Title: Migrating from GTK 3.x to GTK 4
Slug: gtk-migrating-3-to-4

GTK 4 is a major new version of GTK that breaks both API and ABI
compared to GTK 3.x. Thankfully, most of the changes are not hard
to adapt to and there are a number of steps that you can take to
prepare your GTK 3.x application for the switch to GTK 4. After
that, there's a number of adjustments that you may have to do
when you actually switch your application to build against GTK 4.

## Preparation in GTK 3.x

The steps outlined in the following sections assume that your
application is working with GTK 3.24, which is the final stable
release of GTK 3.x. It includes all the necessary APIs and tools
to help you port your application to GTK 4. If you are using
an older version of GTK 3.x, you should first get your application
to build and work with the latest minor release in the 3.24 series.

### Do not use deprecated symbols

Over the years, a number of functions, and in some cases, entire
widgets have been deprecated. These deprecations are clearly spelled
out in the API reference, with hints about the recommended replacements.
The API reference for GTK 3 also includes an
[index](https://developer.gnome.org/gtk3/3.24/api-index-deprecated.html)
of all deprecated symbols.

To verify that your program does not use any deprecated symbols,
you can use defines to remove deprecated symbols from the header files,
as follows:

```
make CFLAGS+="-DGDK_DISABLE_DEPRECATED -DGTK_DISABLE_DEPRECATED"
```

Note that some parts of our API, such as enumeration values, are
not well covered by the deprecation warnings. In most cases, using
them will require you to also use deprecated functions, which will
trigger warnings.

### Enable diagnostic warnings

Deprecations of properties and signals cannot be caught at compile
time, as both properties and signals are installed and used after
types have been instantiated. In order to catch deprecations and
changes in the run time components, you should use the
`G_ENABLE_DIAGNOSTIC` environment variable when running your
application, e.g.:

```
G_ENABLE_DIAGNOSTIC=1 ./your-app
```

### Do not use GTK-specific command line arguments

GTK 4 does not parse command line arguments any more. If you are using
command line arguments like `--gtk-debug` you should use the `GTK_DEBUG`
environment variable instead. If you are using `--g-fatal-warnings` for
debugging purposes, you should use the `G_DEBUG` environment variable, as
specified by the
[GLib documentation](https://developer.gnome.org/glib/stable/glib-running.html).

### Do not use widget style properties

Style properties do not exist in GTK 4. You should stop using them in
your custom CSS and in your code.

### Review your window creation flags

GTK 4 removes the `GDK_WA_CURSOR` flag. Instead, just use
`gdk_window_set_cursor()` to set a cursor on the window after
creating it. GTK 4 also removes the `GDK_WA_VISUAL` flag, and
always uses an RGBA visual for windows. To prepare your code for
this, use `gdk_window_set_visual (gdk_screen_get_rgba_visual ())`
after creating your window. GTK 4 also removes the `GDK_WA_WMCLASS`
flag. If you need this X11-specific functionality, use `XSetClassHint()`
directly.

### Stop using direct access to GdkEvent structs

In GTK 4, event structs are opaque and immutable. Many fields already
have accessors in GTK 3, and you should use those to reduce the amount
of porting work you have to do at the time of the switch.

### Stop using `gdk_pointer_warp()`

Warping the pointer is disorienting and unfriendly to users.
GTK 4 does not support it. In special circumstances (such as when
implementing remote connection UIs) it can be necessary to
warp the pointer; in this case, use platform APIs such as
`XWarpPointer()` directly.

### Stop using non-RGBA visuals

GTK 4 always uses RGBA visuals for its windows; you should make
sure that your code works with such visuals. At the same time,
you should stop using `GdkVisual` APIs, since this object not longer
exists in GTK 4. Most of its APIs are deprecated already and not
useful when dealing with RGBA visuals.

### Stop using `gtk_widget_set_app_paintable`

This is gone in GTK4 with no direct replacement. But for some usecases there
are alternatives. If you want to make the background transparent, you can set
the background color to, for example, `rgba(255, 255, 255, 0)` using CSS instead.

### Stop using `GtkBox` padding, fill and expand child properties

GTK 4 removes these [class@Gtk.Box] child properties, so you should stop using
them. You can replace `GtkBox:padding` using `GtkWidget`'s `margin-*` properties
on your child widgets.

The fill child property can be replaced by setting appropriate values for
the [property@Gtk.Widget:halign] and [property@Gtk.Widget:valign] properties
of the child widgets. If you previously set the `fill` child property to
`TRUE`, you can achieve the same effect by setting the `halign` or `valign`
properties to `GTK_ALIGN_FILL`, depending on the parent box -- `halign` for a
horizontal box, `valign` for a vertical one.

[class@Gtk.Box] also uses the expand child property. It can be replaced by
setting [property@Gtk.Widget:hexpand] or [property@Gtk.Widget:vexpand] on
the child widgets. To match the old behavior of the `GtkBox`'s expand child
property, you need to set `hexpand` on the child widgets of a horizontal
`GtkBox` and `vexpand` on the child widgets of a vertical `GtkBox`.

Note that there's a subtle but important difference between `GtkBox`'s
`expand` and `fill` child properties and the ones in `GtkWidget`: setting
[property@Gtk.Widget:hexpand] or [property@Gtk.Widget:vexpand] to `TRUE`
will propagate up the widget hierarchy, so a pixel-perfect port might
require you to reset the expansion flags to `FALSE` in a parent widget higher
up the hierarchy.

### Stop using the state argument of `GtkStyleContext` getters

The getters in the [class@Gtk.StyleContext] API, such as
`gtk_style_context_get_property()`, `gtk_style_context_get()`,
or `gtk_style_context_get_color()` only accept the context's current
state for their state argument. You should update all callers to pass
the current state.

### Stop using `gdk_pixbuf_get_from_window()` and `gdk_cairo_set_source_window()`

These functions are not supported in GTK 4. Instead, either use
backend-specific APIs, or render your widgets using
[vfunc@Gtk.Widget.snapshot], once you are using GTK 4.

### Stop using `GtkButton`'s image-related API

The functions and properties related to automatically add a
[class@Gtk.Image] to a [class@Gtk.Button], and using a `GtkSetting` to
control its visibility, are not supported in GTK 4. Instead, you can just
pack a GtkImage inside a GtkButton, and control its visibility like you
would for any other widget. If you only want to add a named icon to a
GtkButton, you can use [ctor@Gtk.Button.new_from_icon_name].

### Stop using `GtkWidget` event signals

Event controllers and gestures replace event signals in GTK 4.

Most of them have been backported to GTK 3.x so you can prepare
for this change.

| Signal | Event controller |
| --- | --- |
| ::event | [class@Gtk.EventControllerLegacy] |
| ::event-after | [class@Gtk.EventControllerLegacy] |
| ::button-press-event | [class@Gtk.GestureClick] |
| ::button-release-event | [class@Gtk.GestureClick] |
| ::touch-event | various touch gestures |
| ::scroll-event | [class@Gtk.EventControllerScroll] |
| ::motion-notify-event | [class@Gtk.EventControllerMotion] |
| ::delete-event | - |
| ::key-press-event | [class@Gtk.EventControllerKey] |
| ::key-release-event | [class@Gtk.EventControllerKey] |
| ::enter-notify-event | [class@Gtk.EventControllerMotion] |
| ::leave-notify-event | [class@Gtk.EventControllerMotion] |
| ::configure-event | - |
| ::focus-in-event | [class@Gtk.EventControllerFocus] |
| ::focus-out-event | [class@Gtk.EventControllerFocus] |
| ::map-event | - |
| ::unmap-event | - |
| ::property-notify-event | replaced by [class@Gdk.Clipboard] |
| ::selection-clear-event | replaced by [class@Gdk.Clipboard] |
| ::selection-request-event | replaced by [class@Gdk.Clipboard] |
| ::selection-notify-event | replaced by [class@Gdk.Clipboard] |
| Drag-and-Drop signals | [class@Gtk.DragSource], [class@Gtk.DropTarget] |
| ::proximity-in-event | [class@Gtk.GestureStylus] |
| ::proximity-out-event | [class@Gtk.GestureStylus] |
| ::visibility-notify-event | - |
| ::window-state-event | - |
| ::damage-event | - |
| ::grab-broken-event | - |

Event signals which are not directly related to input have to be dealt with
on a one-by-one basis:

 - If you were using `::configure-event` and `::window-state-event` to save
   window state, you should use property notification for corresponding
   [class@Gtk.Window] properties, such as [property@Gtk.Window:default-width],
   [property@Gtk.Window:default-height], [property@Gtk.Window:maximized] or
   [property@Gtk.Window:fullscreened].
 - If you were using `::delete-event` to present a confirmation when using
   the close button of a window, you should use the
   [signal@Gtk.Window::close-request] signal.
 - If you were using `::map-event` and `::unmap-event` to track a window
   being mapped, you should use property notification for the
   [property@Gdk.Surface:mapped] property instead.
 - The `::damage-event` signal has no replacement, as the only consumer
   of damage events were the offscreen GDK surfaces, which have no
   replacement in GTK 4.x.

### Set a proper application ID

In GTK 4 we want the application's `GApplication` 'application-id' (and
therefore the D-Bus name), the desktop file basename and Wayland's xdg-shell
`app_id` to match. In order to achieve this with GTK 3.x call
`g_set_prgname()` with the same application ID you passed to
[class@Gtk.Application].  Rename your desktop files to match the application
ID if needed.

The call to `g_set_prgname()` can be removed once you fully migrated to GTK 4.

You should be aware that changing the application ID makes your
application appear as a new, different app to application installers.
You should consult the appstream documentation for best practices
around renaming applications.

### Stop using `gtk_main()` and related APIs

GTK 4 removes the `gtk_main_*` family of APIs. The recommended replacement
is [class@Gtk.Application], but you can also iterate the GLib main loop directly,
using `GMainContext` APIs. The replacement for `gtk_events_pending()` is
`g_main_context_pending()`, the replacement for `gtk_main_iteration()` is
`g_main_context_iteration()`.

In GTK 4, you can use this replacement that will iterate the default main loop
until all windows have been closed:

```
while (g_list_model_get_n_items (gtk_window_get_toplevels ()) > 0)
  g_main_context_iteration (NULL, TRUE);
```

### Reduce the use of `gtk_widget_destroy()`

GTK 4 introduces a [method@Gtk.Window.destroy] api. While that is not available
in GTK 3, you can prepare for the switch by using `gtk_widget_destroy()`
only on toplevel windows, and replace all other uses with
`gtk_container_remove()` or `g_object_unref()`.

### Stop using the GtkWidget.destroy vfunc

Instead of implementing GtkWidget.destroy, you can implement GObject.dispose.

### Reduce the use of generic container APIs

GTK 4 removes `gtk_container_add()` and `gtk_container_remove()`. While there
is not always a replacement for `gtk_container_remove()` in GTK 3, you can
replace many uses of `gtk_container_add()` with equivalent container-specific
APIs such as `gtk_box_pack_start()` or `gtk_grid_attach()`, and thereby reduce
the amount of work you have to do at the time of the switch.

### Review your use of icon resources

When using icons as resources, the behavior of GTK 4 is different in one
respect: Icons that are directly in `$APP_ID/icons/` are treated as unthemed
icons, which also means that symbolic icons are not recolored. If you want
your icon resources to have icon theme semantics, they need to be placed
into theme subdirectories such as `$APP_ID/icons/16x16/actions` or
`$APP_ID/icons/scalable/status`.

This location works fine in GTK 3 too, so you can prepare for this change
before switching to GTK 4.

## Changes that need to be done at the time of the switch

This section outlines porting tasks that you need to tackle when
you get to the point that you actually build your application against
GTK 4. Making it possible to prepare for these in GTK 3 would
have been either impossible or impractical.

### Larger changes

Some of the larger themes of GTK 4 development are hard to cover in the form
of checklist items, so we mention them separately up-front.

#### Subclassing

Compared to previous versions, GTK 4 emphasizes composition and delegation
over subclassing. As a consequence, many widgets can no longer be subclassed.
In most cases, you should look deriving your widget directly from GtkWidget
and use complex widgets as child widgets instead of deriving from them.

#### Life-cycle handling

Widgets in GTK 4 are treated like any other objects - their parent widget
holds a reference on them, and GTK holds a reference on toplevel windows.
[method@Gtk.Window.destroy] will drop the reference on the toplevel window,
and cause the whole widget hierarchy to be finalized unless there are other
references that keep widgets alive.

The [signal@Gtk.Widget::destroy] signal is emitted when a widget is
disposed, and therefore can no longer be used to break reference cycles.
A typical sign of a reference cycle involving a toplevel window is when
closing the window does not make the application quit.

### Stop using GdkScreen

The `GdkScreen` object has been removed in GTK 4. Most of its APIs already
had replacements in GTK 3 and were deprecated, a few remaining replacements
have been added to `GdkDisplay`.

### Stop using the root window

The root window is an X11-centric concept that is no longer exposed in the
backend-neutral GDK API. If you need to interact with the X11 root window,
you can use [`method@GdkX11.Display.get_xrootwindow`] to get its XID.

### Stop using `GdkVisual`

This object is not useful with current GTK drawing APIs and has been removed
without replacement.

### Stop using `GdkDeviceManager`

The `GdkDeviceManager` object has been removed in GTK 4. Most of its APIs already
had replacements in GTK 3 and were deprecated in favor of `GdkSeat`.

### Adapt to `GdkWindow` API changes

`GdkWindow` has been renamed to `GdkSurface`.

In GTK 4, the two roles of a standalone toplevel window and of a popup that
is placed relative to a parent window have been separated out into two
interfaces, [iface@Gdk.Toplevel] and [iface@Gdk.Popup]. Surfaces
implementing these interfaces are created with [`ctor@Gdk.Surface.new_toplevel`]
and [`ctor@Gdk.Surface.new_popup`], respectively, and they are presented on
screen using [method@Gdk.Toplevel.present] and [method@Gdk.Popup.present].
The `present()` functions take parameters in the form of an auxiliary layout
struct, [struct@Gdk.PopupLayout] or [struct@Gdk.ToplevelLayout].

If your code is dealing directly with surfaces, you may have to change it
to call the API in these interfaces, depending on whether the surface you are
dealing with is a toplevel or a popup.

As part of this reorganization, X11-only concepts such as sticky,
keep-below, urgency, skip-taskbar or window groups have either been
removed or moved to X11 backend api. If you need to use them on your
X11 windows, you will have to use those backend apis or set the
corresponding X11 properties (as specified in the EWMH) yourself.

Subsurfaces are not currently supported. Native and foreign subwindows
are no longer supported. These concepts were complicating the code
and could not be supported across backends.

A number of GdkWindow APIs are no longer available. This includes
`gdk_window_reparent()`, `gdk_window_set_geometry_hints()`, `gdk_window_raise()`,
`gdk_window_restack()`, `gdk_window_move()`, `gdk_window_resize()`. If
you need to manually control the position or stacking of your X11
windows, you you will have to use Xlib apis.

A number of minor API cleanups have happened in `GdkSurface`
as well. For example, `gdk_surface_input_shape_combine_region()`
has been renamed to [`method@Gdk.Surface.set_input_region`], and
`gdk_surface_begin_resize_drag()` has been renamed to
[`method@Gdk.Toplevel.begin_resize`].

### The "iconified" window state has been renamed to "minimized"

The `GDK_TOPLEVEL_STATE_ICONIFIED` value of the `GdkSurfaceState` enumeration
is now `GDK_TOPLEVEL_STATE_MINIMIZED` in the `GdkToplevelState` enumeration.

The `GdkWindow` functions `gdk_window_iconify()` and
`gdk_window_deiconify()` have been renamed to [method@Gdk.Toplevel.minimize]
and [method@Gdk.Toplevel.present], respectively.

The behavior of the minimization and unminimization operations have
not been changed, and they still require support from the underlying
windowing system.

### Adapt to `GdkEvent` API changes

Direct access to [class@Gdk.Event] structs is no longer possible in GTK 4.
`GdkEvent` is now a strictly read-only type, and you can no longer
change any of its fields, or construct new events. All event fields
have accessors that you will have to use.

Event compression is always enabled in GTK 4, for both motion and
scroll events. If you need to see the uncoalesced motion or scroll
history, use [`method@Gdk.Event.get_history`] on the latest event.

### Stop using grabs

GTK 4 no longer provides the `gdk_device_grab()` or `gdk_seat_grab()`
apis. If you need to dismiss a popup when the user clicks outside
(the most common use for grabs), you can use the `GdkPopup`
[property@Gdk.Popup:autohide] property instead. [class@Gtk.Popover]
also has a [property@Gtk.Popover:autohide] property for this. If you
need to prevent the user from interacting with a window while a dialog
is open, use the [property@Gtk.Window:modal] property of the dialog.

### Adapt to coordinate API changes

A number of coordinate APIs in GTK 3 had `double` variants:
`gdk_device_get_surface_at_position()`, `gdk_surface_get_device_position()`.
These have been changed to use doubles, and the `double` variants
have been removed. Update your code accordingly.

Any APIs that deal with global (or root) coordinates have been
removed in GTK 4, since not all backends support them. You should
replace your use of such APIs with surface-relative equivalents.
Examples of such removed APIs are `gdk_window_get_origin()`,
`gdk_window_move()` or `gdk_event_get_root_coords()`.

### Adapt to `GdkKeymap` API changes

`GdkKeymap` no longer exists as an independent object.

If you need access to keymap state, it is now exposed as properties
on the `GdkDevice` representing the keyboard:
[property@Gdk.Device:direction],
[property@Gdk.Device:has-bidi-layouts],
[property@Gdk.Device:caps-lock-state],
[property@Gdk.Device:num-lock-state],
[property@Gdk.Device:scroll-lock-state] and
[property@Gdk.Device:modifier-state]. To obtain the keyboard device,
you can use

```
gdk_seat_get_keyboard (gdk_display_get_default_seat (display)
```

If you need access to translated keys for event handling, `GdkEvent`
now includes all of the translated key state, including consumed
modifiers, group and shift level, so there should be no need to
manually call `gdk_keymap_translate_keyboard_state()` (which has
been removed).

If you need to do forward or backward mapping between key codes
and key values, use [method@Gdk.Display.map_keycode] and
[method@Gdk.Display.map_keyval], which are the replacements for
`gdk_keymap_get_entries_for_keycode()` and
`gdk_keymap_get_entries_for_keyval()`.

### Adapt to changes in keyboard modifier handling

GTK 3 has the idea that use of modifiers may differ between different
platforms, and has a `GdkModifierIntent` api to let platforms provide
hint about how modifiers are expected to be used. It also promoted
the use of `<Primary>` instead of `<Control>` to specify accelerators that
adapt to platform conventions.

In GTK 4, the meaning of modifiers has been fixed, and backends are
expected to map the platform conventions to the existing modifiers.
The expected use of modifiers in GTK 4 is:

`GDK_CONTROL_MASK`
 : Primary accelerators

`GDK_ALT_MASK`
 :  Mnemonics

`GDK_SHIFT_MASK`
 : Extending selections

`GDK_CONTROL_MASK`
 : Modifying selections

`GDK_CONTROL_MASK|GDK_ALT_MASK`
 : Prevent text input

Consequently, `GdkModifierIntent` and related APIs have been removed,
and `<Control>` is preferred over `<Primary>` in accelerators.

A related change is that GTK 4 no longer supports the use of archaic
X11 'real' modifiers with the names Mod1,..., Mod5, and `GDK_MOD1_MASK`
has been renamed to `GDK_ALT_MASK`.

### Replace `GtkClipboard` with `GdkClipboard`

The `GtkClipboard` API has been removed, and replaced by `GdkClipboard`.
There is not direct 1:1 mapping between the old an the new API, so it cannot
be a mechanical replacement; the new API is based on object types and `GValue`
like object properties, instead of opaque identifiers, so it should be easier
to use.

For instance, the example below copies the contents of an entry into the
clipboard:

```c
static void
copy_text (GtkWidget *widget)
{
  GtkEditable *editable = GTK_EDITABLE (widget);

  // Initialize a GValue with the contents of the widget
  GValue value = G_VALUE_INIT;
  g_value_init (&value, G_TYPE_STRING);
  g_value_set_string (&value, gtk_editable_get_text (editable));

  // Store the value in the clipboard object
  GdkClipboard *clipboard = gtk_widget_get_clipboard (widget);
  gdk_clipboard_set_value (clipboard, &value);

  g_value_unset (&value);
}
```

whereas the example below pastes the contents into the entry:

```c
static void
paste_text (GtkWidget *widget)
{
  GtkEditable *editable = GTK_EDITABLE (widget);

  // Initialize a GValue to receive text
  GValue value = G_VALUE_INIT;
  g_value_init (&value, G_TYPE_STRING);

  // Get the content provider for the clipboard, and ask it for text
  GdkClipboard *clipboard = gtk_widget_get_clipboard (widget);
  GdkContentProvider *provider = gdk_clipboard_get_content (clipboard);

  // If the content provider does not contain text, we are not interested
  if (!gdk_content_provider_get_value (provider, &value, NULL))
    return;

  const char *str = g_value_get_string (&value);

  gtk_editable_set_text (editable, str);

  g_value_unset (&value);
}
```

The convenience API for specific target types in `GtkClipboard` has been
replaced by their corresponding GType:

| GtkClipboard                        | GType                  |
| ----------------------------------- | ---------------------- |
| `gtk_clipboard_request_text()`      | `G_TYPE_STRING`        |
| `gtk_clipboard_request_rich_text()` | `GTK_TYPE_TEXT_BUFFER` |
| `gtk_clipboard_request_image()`     | `GDK_TYPE_PIXBUF`      |
| `gtk_clipboard_request_uris()`      |` GDK_TYPE_FILE_LIST`   |

**Note**: Support for rich text serialization across different processes
for `GtkTextBuffer` is not available any more.

If you are copying the contents of an image, it is recommended to use
`GDK_TYPE_PAINTABLE` instead of `GDK_TYPE_PIXBUF`, to minimize the amount of
potential copies.

### Stop using `gtk_get_current_...` APIs

The function `gtk_get_current_event()` and its variants have been
replaced by equivalent event controller APIs:
[method@Gtk.EventController.get_current_event], etc.

### Convert your UI files

A number of the changes outlined below affect `.ui` files. The
`gtk4-builder-tool simplify` command can perform many of the
necessary changes automatically, when called with the `--3to4`
option. You should always review the resulting changes.

The `<requires>` tag now supports for the `lib` attribute the
`gtk` value only, instead of the `gtk+` one previously.

### Adapt to GtkBuilder API changes

`gtk_builder_connect_signals()` no longer exists. Instead, signals are
always connected automatically. If you need to add user data to your
signals, [method@Gtk.Builder.set_current_object] must be called. An important
caveat is that you have to do this before loading any XML. This means if
you need to use [method@Gtk.Builder.set_current_object], you can no longer use
[ctor@Gtk.Builder.new_from_file], [ctor@Gtk.Builder.new_from_resource], or
[ctor@Gtk.Builder.new_from_string]. Instead, you must use vanilla [ctor@Gtk.Builder.new],
then call [method@Gtk.Builder.set_current_object], then load the XML using
[method@Gtk.Builder.add_from_file], [method@Gtk.Builder.add_from_resource], or
[method@Gtk.Builder.add_from_string]. You must check the return value for
failure and manually abort with `g_error()` if something went wrong.

You only have to worry about this if you were previously using
`gtk_builder_connect_signals()`. If you are using templates, then
`gtk_widget_init_template()` will call `gtk_builder_set_current_object()`
for you, so templates work like before.

### Adapt to event controller API changes

A few changes to the event controller and [class@Gtk.Gesture] APIs
did not make it back to GTK 3, and have to be taken into account
when moving to GTK 4. One is that the [signal@Gtk.EventControllerMotion::enter]
and [signal@Gtk.EventControllerMotion::leave] signals have gained new arguments.
Another is that `GtkGestureMultiPress` has been renamed to [class@Gtk.GestureClick],
and has lost its area property. A [class@Gtk.EventControllerFocus] has been
split off from [class@Gtk.EventControllerKey].

In GTK 3, `GtkEventController:widget` was a construct-only property, so a
`GtkWidget` was provided whenever constructing a `GtkEventController`. In
GTK 4, [property@Gtk.EventController:widget] is now read-only. Use
[method@Gtk.Widget.add_controller] to add an event controller to a widget.

In GTK 3, widgets did not own their event controllers, and event
controllers did not own their widgets, so developers were responsible
for manually keeping event controllers alive for the lifetime of their
associated widgets. In GTK 4, widgets own their event controllers.
[method@Gtk.Widget.add_controller] takes ownership of the event controller, so
there is no longer any need to store a reference to the event controller
after it has been added to a widget.

Although not normally needed, an event controller could be removed from
a widget in GTK 3 by destroying the event controller with `g_object_unref()`.
In GTK 4, you must use [method@Gtk.Widget.remove_controller].

### Focus handling changes

The semantics of the [property@Gtk.Widget:can-focus] property have changed.
In GTK 3, this property only meant that the widget itself would not
accept keyboard input, but its children still might (in the case of
containers). In GTK 4, if `:can-focus` is `FALSE`, the focus cannot enter
the widget or any of its descendents, and the default value has changed
from `FALSE` to `TRUE`. In addition, there is a [property@Gtk.Widget:focusable]
property, which controls whether an individual widget can receive
the input focus.

The `gtk4-builder-tool` utility, when called with the `--3to4` option of the
`simplify` command, will replace `:can-focus` by `:focusable`.

The feature to automatically keep the focus widget scrolled into view with
`gtk_container_set_focus_vadjustment()` has been removed together with
`GtkContainer`, and is provided by scrollable widgets instead. In the common
case that the scrollable is a [class@Gtk.Viewport], use
[property@Gtk.Viewport:scroll-to-focus].

### Use the new apis for keyboard shortcuts

The APIs for keyboard shortcuts and accelerators have changed in GTK 4.

Instead of `GtkAccelGroup`, you now use a [class@Gtk.ShortcutController] with global
scope, and instead of `GtkBindingSet`, you now use [method@Gtk.WidgetClass.add_shortcut],
[method@Gtk.WidgetClass.add_binding] and its variants. In both cases, you probably
want to add actions that can be triggered by your shortcuts.

There is no direct replacement for loading and saving accelerators with
`GtkAccelMap`. But since `GtkShortcutController` implements `GListModel` and
both [class@Gtk.ShortcutTrigger] and [class@Gtk.ShortcutAction] can be
serialized to strings, it is relatively easy to implement saving and loading
yourself.

### Stop using `GtkEventBox`

`GtkEventBox` is no longer needed and has been removed.

All widgets receive all events.

### Stop using `GtkButtonBox`

`GtkButtonBox` has been removed. Use a [class@Gtk.Box] instead.

### Adapt to `GtkBox` API changes

The GtkBox `pack_start()` and `pack_end()` methods have been replaced by
[method@Gtk.Box.prepend] and [method@Gtk.Box.append]. You can also reorder
box children as necessary.

### Adapt to `GtkWindow` API changes

Following the `GdkSurface` changes, a number of `GtkWindow` APIs that were
X11-specific have been removed. This includes `gtk_window_set_position()`,
`gtk_window_set_geometry_hints()`, `gtk_window_set_gravity()`,
`gtk_window_move()`, `gtk_window_parse_geometry()`,
`gtk_window_set_keep_above()`, `gtk_window_set_keep_below()`,
`gtk_window_begin_resize_drag()`, `gtk_window_begin_move_drag()`.
Most likely, you should just stop using them. In some cases, you can
fall back to using the underlying `GdkToplevel` APIs (for example,
[`method@Gdk.Toplevel.begin_resize`]); alternatively, you will need to get
the native windowing system surface from the `GtkWindow` and call platform
specific API.

The APIs for controlling `GtkWindow` size have changed to be better aligned
with the way size changes are integrated in the frame cycle. `gtk_window_resize()`
and `gtk_window_get_size()` have been removed. Instead, use
[`method@Gtk.Window.set_default_size`] and [`method@Gtk.Window.get_default_size`].

### Adapt to `GtkHeaderBar` and `GtkActionBar` API changes

The `gtk_header_bar_set_show_close_button()` function has been renamed to
the more accurate name [method@Gtk.HeaderBar.set_show_title_buttons]. The
corresponding getter and the property itself have also been renamed.
The default value of the property is now `TRUE` instead of `FALSE`.

The `gtk_header_bar_set_custom_title()` function has been renamed to
the more accurate name [method@Gtk.HeaderBar.set_title_widget]. The
corresponding getter and the property itself have also been renamed.

The `gtk_header_bar_set_title()` function has been removed along with its
corresponding getter and the property. By default [class@Gtk.HeaderBar]
shows the title of the window, so if you were setting the title of the
header bar, consider setting the window title instead. If you need to show a
title that's different from the window title, use the
[property@Gtk.HeaderBar:title-widget] property to add a [class@Gtk.Label] as
shown in the example in the [class@Gtk.HeaderBar] documentation.

The `gtk_header_bar_set_subtitle()` function has been removed along with
its corresponding getter and the property. The old "subtitle" behavior
can be replicated by setting the [property@Gtk.HeaderBar:title-widget] property to
a [class@Gtk.Box] with two labels inside, with the title label matching the
example in `GtkHeaderBar` documentation, and the subtitle label being
similar, but with `"subtitle"` style class instead of `"title"`.

The `gtk_header_bar_set_has_subtitle()` function has been removed along with
its corresponding getter and the property. Its behavior can be replicated by
setting the [property@Gtk.HeaderBar:title-widget] property to a
[class@Gtk.Stack] with [property@Gtk.Stack:vhomogeneous] property set to
`TRUE` and two pages, each with a [class@Gtk.Box] with title and subtitle as
described above.

Some of the internal structure of `GtkHeaderBar` has been made available as
public API: [class@Gtk.WindowHandle] and [class@Gtk.WindowControls]. If you
have unusual needs for custom headerbars, these might be useful to you.

The `:pack-type` child properties of `GtkHeaderBar` and `GtkActionBar` have
been removed. If you need to programmatically place children, use the
[method@Gtk.HeaderBar.pack_start] and [method@Gtk.HeaderBar.pack_end] methods.
In UI files, use the `type` attribute on the `child` element.

The `gtk4-builder-tool` utility can help with this conversion, with the
`--3to4` option of the `simplify` command.

### Adapt to GtkStack, GtkAssistant and GtkNotebook API changes

The child properties of `GtkStack`, `GtkAssistant` and `GtkNotebook` have been
converted into child meta objects.

Instead of `gtk_container_child_set (stack, child, …)`, you can now use
`g_object_set (gtk_stack_get_page (stack, child), …)`. In .ui files, the
`GtkStackPage` objects must be created explicitly, and take the child widget
as property. The changes to `GtkNotebook` and `GtkAssistant` are similar.

`gtk4-builder-tool` can help with this conversion, with the `--3to4` option
of the `simplify` command.

### Adapt to button class hierarchy changes

`GtkCheckButton` is no longer derived from `GtkToggleButton`. Call
[method@Gtk.CheckButton.set_active] instead of [method@Gtk.ToggleButton.set_active].

`GtkRadioButton` has been removed, and its grouping functionality has
been added to `GtkCheckButton` and `GtkToggleButton`. Use grouped
check buttons for traditional radio groups, and used grouped toggle
buttons for view switchers. The new API to set up groups of buttons
is [method@Gtk.CheckButton.set_group] and [method@Gtk.ToggleButton.set_group].

`gtk4-builder-tool` can help with this conversion, with the `--3to4` option
of the `simplify` command.

### Adapt to GtkScrolledWindow API changes

The constructor for `GtkScrolledWindow` no longer takes the adjustments
as arguments - these were almost always `NULL`.

### Adapt to GtkBin removal

The abstract base class `GtkBin` for single-child containers has been
removed. The former subclasses are now derived directly from `GtkWidget`,
and have a "child" property for their child widget. To add a child, use
the setter for the "child" property (e.g. [method@Gtk.Frame.set_child]) instead
of `gtk_container_add()`. Adding a child in a ui file with `<child>` still works.

The affected classes are:

- [class@Gtk.AspectFrame]
- [class@Gtk.Button] (and subclasses)
- [class@Gtk.ComboBox]
- [class@Gtk.FlowBoxChild]
- [class@Gtk.Frame]
- [class@Gtk.ListBoxRow]
- [class@Gtk.Overlay]
- [class@Gtk.Popover]
- [class@Gtk.Revealer]
- [class@Gtk.ScrolledWindow]
- [class@Gtk.SearchBar]
- [class@Gtk.Viewport]
- [class@Gtk.Window] (and subclasses)

If you have custom widgets that were derived from `GtkBin`, you should
port them to derive from `GtkWidget`. Notable vfuncs that you will have
to implement include the `GObject` dispose vfunc (to unparent your child),
[vfunc@Gtk.Widget.compute_expand] (if you want your container to propagate
expand flags) and [vfunc@Gtk.Widget.get_request_mode] (if you want your
container to support height-for-width).

You may also want to implement the [iface@Gtk.Buildable] interface, to support
adding children with `<child>` in ui files.

### Adapt to GtkContainer removal

The abstract base class `GtkContainer` for general containers has been
removed. The former subclasses are now derived directly from `GtkWidget`,
and have class-specific add() and remove() functions.

The most noticeable change is the use of [method@Gtk.Box.append] or [method@Gtk.Box.prepend]
instead of `gtk_container_add()` for adding children to `GtkBox`, and the change
to use container-specific remove functions, such as [method@Gtk.Stack.remove] instead
of `gtk_container_remove()`. Adding a child in a ui file with `<child>` still works.

The affected classes are:

- [class@Gtk.ActionBar]
- [class@Gtk.Box] (and subclasses)
- [class@Gtk.Expander]
- [class@Gtk.Fixed]
- [class@Gtk.FlowBox]
- [class@Gtk.Grid]
- [class@Gtk.HeaderBar]
- [class@Gtk.IconView]
- [class@Gtk.InfoBar]
- [class@Gtk.ListBox]
- [class@Gtk.Notebook]
- [class@Gtk.Paned]
- [class@Gtk.Stack]
- [class@Gtk.TextView]
- [class@Gtk.TreeView]

Without `GtkContainer`, there are no longer facilities for defining and
using child properties. If you have custom widgets using child properties,
they will have to be converted either to layout properties provided
by a layout manager (if they are layout-related), or handled in some
other way. One possibility is to use child meta objects, as seen with
[class@Gtk.AssistantPage], [class@Gtk.StackPage] and the like.

If you used to define child properties with `<packing>` in ui files, you have
to switch to using `<layout>` for the corresponding layout properties.
`gtk4-builder-tool` can help with this conversion, with the `--3to4` option
of the `simplify` command.

The replacements for gtk_container_add() are:

| Widget | Replacement |
| ------ | ----------- |
| `GtkActionBar`    | [method@Gtk.ActionBar.pack_start], [method@Gtk.ActionBar.pack_end] |
| `GtkBox`          | [method@Gtk.Box.prepend], [method@Gtk.Box.append] |
| `GtkExpander`     | [method@Gtk.Expander.set_child] |
| `GtkFixed`        | [method@Gtk.Fixed.put] |
| `GtkFlowBox`      | [method@Gtk.FlowBox.insert] |
| `GtkGrid`         | [method@Gtk.Grid.attach] |
| `GtkHeaderBar`    | [method@Gtk.HeaderBar.pack_start], [method@Gtk.HeaderBar.pack_end] |
| `GtkIconView`     | - |
| `GtkInfoBar`      | [method@Gtk.InfoBar.add_child] |
| `GtkListBox`      | [method@Gtk.ListBox.insert] |
| `GtkNotebook`     | [method@Gtk.Notebook.append_page] |
| `GtkPaned`        | [method@Gtk.Paned.set_start_child], [method@Gtk.Paned.set_end_child] |
| `GtkStack`        | [method@Gtk.Stack.add_child] |
| `GtkTextView`     | [method@Gtk.TextView.add_child_at_anchor], [method@Gtk.TextView.add_overlay] |
| `GtkTreeView`     | - |

### Stop using GtkContainer::border-width

GTK 4 has removed the `GtkContainer::border-width` property (together
with the rest of `GtkContainer`). Use other means to influence the spacing
of your containers, such as the CSS margin and padding properties on child
widgets, or the CSS border-spacing property on containers.

### Adapt to gtk_widget_destroy() removal

The function `gtk_widget_destroy()` has been removed. To explicitly destroy
a toplevel window, use [method@Gtk.Window.destroy]. To destroy a widget that is
part of a hierarchy, remove it from its parent using a container-specific
remove API, such as [method@Gtk.Box.remove] or [method@Gtk.Stack.remove]. To
destroy a freestanding non-toplevel widget, use `g_object_unref()` to drop your
reference.

### Adapt to coordinate API changes

A number of APIs that are accepting or returning coordinates have
been changed from ints to doubles: `gtk_widget_translate_coordinates()`,
`gtk_fixed_put()`, `gtk_fixed_move()`. This change is mostly transparent,
except for cases where out parameters are involved: you need to
pass double* now, instead of int*.

### Adapt to GtkStyleContext API changes

The getters in the GtkStyleContext API, such as
[method@Gtk.StyleContext.get_color], [method@Gtk.StyleContext.get_border],
or [method@Gtk.StyleContext.get_margin] have lost their state argument,
and always use the context's current state. Update all callers
to omit the state argument.

The most commonly used GtkStyleContext API, `gtk_style_context_add_class()`,
has been moved to GtkWidget as [method@Gtk.Widget.add_css_class], as have the
corresponding `gtk_style_context_remove_class()` and
`gtk_style_context_has_class()` APIs.

### Adapt to GtkCssProvider API changes

In GTK 4, the various `GtkCssProvider` load functions have lost their
`GError` argument. If you want to handle CSS loading errors, use the
[signal@Gtk.CssProvider::parsing-error] signal instead. gtk_css_provider_get_named()
has been replaced by [method@Gtk.CssProvider.load_named].

### Stop using GtkShadowType and GtkRelief properties

The shadow-type properties in `GtkScrolledWindow`, `GtkViewport`,
and `GtkFrame`, as well as the relief properties in `GtkButton`
and its subclasses have been removed. `GtkScrolledWindow`, `GtkButton`
and `GtkMenuButton` have instead gained a boolean has-frame property.

### Adapt to GtkWidget's size request changes

GTK 3 used five different virtual functions in GtkWidget to
implement size requisition, namely the `gtk_widget_get_preferred_width()`
family of functions. To simplify widget implementations, GTK 4 uses
only one virtual function, [vfunc@Gtk.Widget.measure], that widgets
have to implement. [method@Gtk.Widget.measure] replaces the various
`gtk_widget_get_preferred_` functions for querying sizes.

### Adapt to GtkWidget's size allocation changes

The [vfunc@Gtk.Widget.size_allocate] vfunc takes the baseline as an argument
now, so you no longer need to call `gtk_widget_get_allocated_baseline()`
to get it.

The ::size-allocate signal has been removed, since it is easy
to misuse. If you need to learn about sizing changes of custom
drawing widgets, use the [signal@Gtk.DrawingArea::resize] or
[signal@Gtk.GLArea::resize] signals. If you want to track the size
of toplevel windows, use property notification for
[property@Gtk.Window:default-width] and [property@Gtk.Window:default-height].

### Switch to GtkWidget's children APIs

In GTK 4, any widget can have children (and `GtkContainer` is gone).
There is new API to navigate the widget tree, for use in widget
implementations:
[method@Gtk.Widget.get_first_child],
[method@Gtk.Widget.get_last_child],
[method@Gtk.Widget.get_next_sibling],
[method@Gtk.Widget.get_prev_sibling].

### Don't use -gtk-gradient in your CSS

GTK now supports standard CSS syntax for both linear and radial
gradients, just use those.

### Don't use -gtk-icon-effect in your CSS

GTK now supports a more versatile -gtk-icon-filter instead.

Replace

| Old | Replacement |
| ------ | ----------- |
| -gtk-icon-effect: dim | -gtk-icon-filter: opacity(0.5) |
| -gtk-icon-effect: highlight | -gtk-icon-filter: brightness(1.2) |

### Don't use -gtk-icon-theme in your CSS

GTK 4 always uses the current icon theme, with no way to change this.

### Don't use -gtk-outline-...-radius in your CSS

These non-standard properties have been removed from GTK
CSS. Just use regular border radius.

### Adapt to drawing model changes

This area has seen the most radical changes in the transition from GTK 3
to GTK 4. Widgets no longer use a draw() function to render their contents
to a cairo surface. Instead, they have a [vfunc@Gtk.Widget.snapshot] function
that creates one or more GskRenderNodes to represent their content. Third-party
widgets that use a draw() function or a `GtkWidget::draw` signal handler for
custom drawing will need to be converted to use [method@Gtk.Snapshot.append_cairo].

The auxiliary [class@Gtk.Snapshot] object has APIs to help with creating render
nodes.

If you are using a `GtkDrawingArea` for custom drawing, you need to switch
to using [method@Gtk.DrawingArea.set_draw_func] to set a draw function instead
of connecting a handler to the `GtkWidget::draw` signal.

### Stop using APIs to query GdkSurfaces

A number of APIs for querying special-purpose windows have been removed,
since these windows no longer exist:
`gtk_tree_view_get_bin_window()`, `gtk_viewport_get_bin_window()`,
`gtk_viewport_get_view_window()`.

### Widgets are now visible by default

The default value of [property@Gtk.Widget:visible] in GTK 4 is %TRUE, so you no
longer need to explicitly show all your widgets. On the flip side, you
need to hide widgets that are not meant to be visible from the start.
The only widgets that still need to be explicitly shown are toplevel
windows, dialogs and popovers.

A convenient way to remove unnecessary property assignments like this
from ui files it run the command `gtk4-builder-tool simplify --replace`
on them.

The function `gtk_widget_show_all()`, the `GtkWidget:no-show-all` property
and its getter and setter have been removed in GTK 4, so you should stop
using them.

### Adapt to changes in animated hiding and showing of widgets

Widgets that appear and disappear with an animation, such as
`GtkInfoBar`, `GtkRevealer` no longer use `gtk_widget_show()` and
`gtk_widget_hide()` for this, but have gained dedicated APIs for this
purpose that you should use instead, such as [method@Gtk.InfoBar.set_revealed].

### Stop passing commandline arguments to gtk_init

The [func@Gtk.init] and [func@Gtk.init_check] functions no longer accept
commandline arguments. Just call them without arguments. Other initialization
functions that were purely related to commandline argument handling, such as
`gtk_parse_args()` and `gtk_get_option_group()`, are gone.

The APIs to initialize GDK separately are also gone, but it is very unlikely
that you are affected by that.

### GdkPixbuf is deemphasized

A number of `GdkPixbuf`-based APIs have been removed. The available replacements
are either using `GIcon`, or the newly introduced [class@Gdk.Texture] or
[iface@Gdk.Paintable] classes instead. If you are dealing with pixbufs, you can use
[ctor@Gdk.Texture.new_for_pixbuf] to convert them to texture objects where needed.

### GtkWidget event signals are removed

Event controllers and GtkGestures have already been introduced in GTK 3 to handle
input for many cases. In GTK 4, the traditional widget signals for handling input,
such as `GtkWidget::motion-event` or `GtkWidget::event` have been removed. All event
handling is done via event controllers now.

### Invalidation handling has changed

Only [method@Gtk.Widget.queue_draw] is left to mark a widget as needing redraw.
Variations like `gtk_widget_queue_draw_rectangle()` or `gtk_widget_queue_draw_region()`
are no longer available.

### Stop using GtkWidget::draw

The `GtkWidget::draw` signal has been removed. Widgets need to implement the
[vfunc@Gtk.Widget.snapshot] function now. Connecting draw signal handlers is
no longer possible. If you want to keep using cairo for drawing, use
[method@Gtk.Snapshot.append_cairo].

### Window content observation has changed

Observing widget contents and widget size is now done by using the
[class@Gtk.WidgetPaintable] object instead of connecting to widget signals.

### Monitor handling has changed

Instead of a monitor number, [class@Gdk.Monitor] is now used throughout.
[method@Gdk.Display.get_monitors] returns the list of monitors that can be queried
or observed for monitors to pass to APIs like [method@Gtk.Window.fullscreen_on_monitor].

### Adapt to monitor API changes

The `gdk_monitor_get_workarea()` API is gone. Individual backends can still
provide this information, for example with [method@GdkX11.Monitor.get_workarea].

If you use this information, your code should check which backend is in
use and then call the appropriate backend API.

### Adapt to cursor API changes

Use the new [method@Gtk.Widget.set_cursor] function to set cursors, instead of
setting the cursor on the underlying window directly. This is necessary
because most widgets don't have their own window anymore, turning any
such calls into global cursor changes.

For creating standard cursors, `gdk_cursor_new_for_display()` has been removed,
you have to use cursor names instead of `GdkCursorType`. For creating custom cursors,
use [ctor@Gdk.Cursor.new_from_texture]. The ability to get cursor images has been removed.

### Adapt to icon size API changes

Instead of the existing extensible set of symbolic icon sizes, GTK now only
supports normal and large icons with the [enum@Gtk.IconSize] enumeration. The actual sizes
can be defined by themes via the CSS property -gtk-icon-size.

GtkImage setters like [method@Gtk.Image.set_from_icon_name] no longer take a `GtkIconSize`
argument. You can use the separate [method@Gtk.Image.set_icon_size] setter if you need
to override the icon size.

The :stock-size property of GtkCellRendererPixbuf has been renamed to
[property@Gtk.CellRendererPixbuf:icon-size].

### Adapt to changes in the GtkAssistant API

The :has-padding property is gone, and `GtkAssistant` no longer adds padding
to pages. You can easily do that yourself.

### Adapt to changes in the API of GtkEntry, GtkSearchEntry and GtkSpinButton

The [iface@Gtk.Editable] interface has been made more useful, and the core functionality of
`GtkEntry` has been broken out as a [class@Gtk.Text] widget.
[class@Gtk.Entry],
[class@Gtk.SearchEntry],
[class@Gtk.SpinButton] and the new
[class@Gtk.PasswordEntry] now use a [class@Gtk.Text] widget internally
and implement [iface@Gtk.Editable]. In particular, this means that it is no longer
possible to use `GtkEntry` API such as `gtk_entry_grab_focus_without_selecting()`
on a search entry.

Use `GtkEditable` API for editable functionality, and widget-specific APIs for
things that go beyond the common interface. For password entries, use
[class@Gtk.PasswordEntry]. As an example, `gtk_spin_button_set_max_width_chars()`
has been removed in favor of [method@Gtk.Editable.set_max_width_chars].

### Adapt to changes in GtkOverlay API

The GtkOverlay :pass-through child property has been replaced by the
[property@Gtk.Widget:can-target] property. Note that they have the opposite sense:
pass-through == !can-target.

### Use GtkFixed instead of GtkLayout

Since `GtkScrolledWindow` can deal with widgets that do not implement
the `GtkScrollable` interface by automatically wrapping them into a
`GtkViewport`, `GtkLayout` is redundant, and has been removed in favor
of the existing [class@Gtk.Fixed] widget.

### Adapt to search entry changes

The way search entries are connected to global events has changed;
`gtk_search_entry_handle_event()` has been dropped and replaced by
[method@Gtk.SearchEntry.set_key_capture_widget] and
[method@Gtk.EventControllerKey.forward].

### Adapt to GtkScale changes

The default value of `GtkScale:draw-value` has been changed to %FALSE.
If you want your scales to draw values, you will have to set this
property explicitly now.

`gtk4-builder-tool` can help with this conversion, with the `--3to4` option
of the `simplify` command.

### Stop using gtk_window_activate_default()

The handling of default widgets has been changed, and activating
the default now works by calling [method@Gtk.Widget.activate_default]
on the widget that caused the activation. If you have a custom widget
that wants to override the default handling, you can provide an
implementation of the "default.activate" action in your widgets' action
groups.

### Stop using gtk_widget_grab_default()

The function `gtk_widget_grab_default()` has been removed. If you need
to mark a widget as default, use [method@Gtk.Window.set_default_widget]
directly.

### Stop setting ::has-default and ::has-focus in .ui files

The special handling for the :has-default and :has-focus properties
has been removed. If you want to define the initial focus or the
the default widget in a .ui file, set the [property@Gtk.Window:default-widget] or
[property@Gtk.Window:focus-widget] properties of the toplevel window.

### Stop using the GtkWidget::display-changed signal

To track the current display, use the [property@Gtk.Widget:root] property instead.

### GtkPopover::modal has been renamed to autohide

The modal property has been renamed to [property@Gtk.Popover:autohide].

`gtk-builder-tool` can assist with the rename in ui files.

### gtk_widget_get_surface has been removed

`gtk_widget_get_surface()` has been removed.
Use [method@Gtk.Native.get_surface] in combination with
[method@Gtk.Widget.get_native] instead.

### gtk_widget_is_toplevel has been removed

`gtk_widget_is_toplevel()` has been removed.
Use `GTK_IS_ROOT`, `GTK_IS_NATIVE` or `GTK_IS_WINDOW`
instead, as appropriate.

### gtk_widget_get_toplevel has been removed

`gtk_widget_get_toplevel()` has been removed.
Use [method@Gtk.Widget.get_root] or [method@Gtk.Widget.get_native]
instead, as appropriate.

### GtkEntryBuffer ::deleted-text has changed

To allow signal handlers to access the deleted text before it
has been deleted, the [signal@Gtk.EntryBuffer::deleted-text] signal
has changed from %G_SIGNAL_RUN_FIRST to %G_SIGNAL_RUN_LAST. The default
handler removes the text from the [class@Gtk.EntryBuffer].

To adapt existing code, use `g_signal_connect_after()` or
%G_CONNECT_AFTER when using `g_signal_connect_data()` or
`g_signal_connect_object()`.

### GtkMenu, GtkMenuBar and GtkMenuItem are gone

These widgets were heavily relying on X11-centric concepts such as
override-redirect windows and grabs, and were hard to adjust to other
windowing systems.

Menus can already be replaced using GtkPopoverMenu in GTK 3. Additionally,
GTK 4 introduces GtkPopoverMenuBar to replace menubars. These new widgets
can only be constructed from menu models, so the porting effort involves
switching to menu models and actions.

Tabular menus were rarely used and complicated the menu code,
so they have not been brought over to [class@Gtk.PopoverMenu].
If you need complex layout in menu-like popups, consider directly using a
[class@Gtk.Popover] instead.

Since menus are gone, `GtkMenuButton` also lost its ability to show menus,
and needs to be used with popovers in GTK 4.

### GtkToolbar has been removed

Toolbars were using outdated concepts such as requiring special toolitem
widgets. Toolbars should be replaced by using a `GtkBox` with regular widgets
instead and the "toolbar" style class.

### GtkAspectFrame is no longer a frame

`GtkAspectFrame` is no longer derived from `GtkFrame` and does not
place a label and frame around its child anymore. It still lets
you control the aspect ratio of its child.

### Stop using custom tooltip windows

Tooltips no longer use `GtkWindow`s in GTK 4, and it is no longer
possible to provide a custom window for tooltips. Replacing the content
of the tooltip with a custom widget is still possible, with
[method@Gtk.Tooltip.set_custom].

### Switch to the new Drag-and-Drop api

The source-side Drag-and-Drop apis in GTK 4 have been changed to use an event
controller, [class@Gtk.DragSource]. Instead of calling `gtk_drag_source_set()`
and connecting to `GtkWidget` signals, you create a [class@Gtk.DragSource] object,
attach it to the widget with [method@Gtk.Widget.add_controller], and connect
to `GtkDragSource` signals. Instead of calling `gtk_drag_begin()` on a widget
to start a drag manually, call [func@Gdk.Drag.begin].
The `::drag-data-get` signal has been replaced by the [signal@Gtk.DragSource::prepare]
signal, which returns a [class@Gdk.ContentProvider] for the drag operation.

The destination-side Drag-and-Drop API in GTK 4 have also been changed
to use an event controller, [class@Gtk.DropTarget]. Instead of calling
`gtk_drag_dest_set()` and connecting to `GtkWidget` signals, you create
a [class@Gtk.DropTarget] object, attach it to the widget with
[method@Gtk.Widget.add_controller], and connect to `GtkDropTarget` signals.
The `::drag-motion` signal has been renamed to [signal@Gtk.DropTarget::accept],
and instead of `::drag-data-received`, you need to use async read methods on the
[class@Gdk.Drop] object, such as [method@Gdk.Drop.read_async] or
[method@Gdk.Drop.read_value_async].

### Adapt to GtkIconTheme API changes

`gtk_icon_theme_lookup_icon()` returns a [class@Gtk.IconPaintable] object now,
instead of a `GtkIconInfo`. It always returns a paintable in the requested size,
and never fails. A number of no-longer-relevant lookup flags and API variants
have been removed.

Note that while GTK 4 is moving towards [iface@Gdk.Paintable] as a primary API
for paintable content, it is meant to be a 'pure' content producer, therefore
a [class@Gtk.IconPaintable] for a symbolic icon will *not* get recolored depending
on the context it is rendered it. To properly render a symbolic icon that
is provided in the form of a `GtkIconPaintable` (this can be checked with
[method@Gtk.IconPaintable.is_symbolic]), you have to call
[method@Gtk.IconPaintable.get_icon_name] and set the icon name on a `GtkImage`.

### Adapt to GtkImage changes
`GtkPicture`'s behaviour was "split out" of `GtkImage` as the latter was covering
too many use cases; if you're loading an icon, [class@Gtk.Image] in GTK3 and GTK4 are
perfectly equivalent. If you are loading a more complex image asset, like a picture
or a thumbnail, then [class@Gtk.Picture] is the appropriate widget.

One noteworthy distinction is that while `GtkImage` has its size computed by
GTK, `GtkPicture` lets you decide about the size.

### Update to GtkFileChooser API changes

`GtkFileChooser` moved to a GFile-based API. If you need to convert a path
or a URI, use `g_file_new_for_path()`, `g_file_new_for_commandline_arg()`,
or `g_file_new_for_uri()`; similarly, if you need to get a path, name or URI
from a `GFile`, use `g_file_get_path()`, `g_file_get_basename()` or `g_file_get_uri()`.
With the removal or path and URI-based functions, the "local-only" property
has been removed; GFile can be used to access non-local as well as local
resources.

The `GTK_FILE_CHOOSER_ACTION_CREATE_FOLDER` action has been removed. Use
%GTK_FILE_CHOOSER_ACTION_SELECT_FOLDER, instead. If a new folder is needed,
the user can create one.

The "confirm-overwrite" signal, and the "do-overwrite-confirmation"
property have been removed from `GtkFileChooser`. The file chooser widgets
will automatically handle the confirmation of overwriting a file when
using `GTK_FILE_CHOOSER_ACTION_SAVE`.

`GtkFileChooser` does not support a custom extra widget any more. If you
need to add extra widgets, use [method@Gtk.FileChooser.add_choice] instead.

`GtkFileChooser` does not support a custom preview widget any more.

### Stop using blocking dialog functions

`GtkDialog`, `GtkNativeDialog`, and `GtkPrintOperation` removed their
blocking API using nested main loops. Nested main loops present
re-entrancy issues and other hard to debug issues when coupled
with other event sources (IPC, accessibility, network operations)
that are not under the toolkit or the application developer's
control. Additionally, "stop-the-world" functions do not fit
the event-driven programming model of GTK.

You can replace calls to `gtk_dialog_run()` by specifying that the
`GtkDialog` must be modal using [method@Gtk.Window.set_modal] or the
%GTK_DIALOG_MODAL flag, and connecting to the [signal@Gtk.Dialog::response]
signal.

### Stop using GtkBuildable API

All the `GtkBuildable` API was made private, except for the
getter function to retrieve the buildable ID. If you are
using `gtk_buildable_get_name()` you should replace it with
[method@Gtk.Buildable.get_buildable_id].

### Adapt to GtkAboutDialog API changes

`GtkAboutDialog` now directly derives from `GtkWindow`, the `GtkDialog`
API can no longer be used on it.

### Adapt to GtkTreeView and GtkIconView tooltip context changes

The getter functions for retrieving the data from `GtkTreeView`
and `GtkIconView` inside a `GtkWidget::query-tooltip` signal do not take the
pointer coordinates as inout arguments any more, but as normal in ones.

See: [method@Gtk.TreeView.get_tooltip_context], [method@Gtk.IconView.get_tooltip_context]

### Adapt to GtkPopover changes

In GTK 3, a `GtkPopover` could be attached to any widget, using the `relative-to`
property. This is no longer possible in GTK 4. The parent widget has to be aware
of its popover children, and manage their size allocation. Therefore, only widgets
with dedicated popover support can have them, such as [class@Gtk.MenuButton] or
[class@Gtk.PopoverMenuBar].

If you want to make a custom widget that has an attached popover, you need to call
[method@Gtk.Popover.present] in your [vfunc@Gtk.Widget.size_allocate] vfunc, in order
to update the positioning of the popover.

### Stop using GtkFileChooserButton

The `GtkFileChooserButton` widget was removed, due to its shortcomings in
the user interaction. You can replace it with a simple `GtkButton` that
shows a [class@Gtk.FileChooserNative] dialog when clicked; once the file selection
has completed, you can update the label of the `GtkButton` with the selected
file.

### Adapt to changed GtkSettings properties

In GTK 3 the [property@Gtk.Settings:gtk-cursor-aspect-ratio] property of
`GtkSettings` was a `float`. In GTK 4 this has been changed to a `double`.

## Changes to consider after the switch

GTK 4 has a number of new features that you may want to take
advantage of once the dust has settled over the initial migration.

### Consider porting to the new list widgets

In GTK 2 and 3, `GtkTreeModel` and `GtkCellRenderer` and widgets using
these were the primary way of displaying data and lists. GTK 4 brings
a new family of widgets for this purpose that uses list models instead
of tree models, and widgets instead of cell renderers.

To learn more about the new list widgets, you can read the [List Widget
Overview](https://docs.gtk.org/gtk4/section-list-widget.html).

