Documentation: Add GTK UI build prerequisites and style guide

Add GTK4/libadwaita build instructions alongside Qt.
Add Documentation/GtkFrontend.md with GTK-specific coding style
rules and a manual test checklist.

Documentation/BuildInstructionsLadybird.md

@@ -252,14 +252,35 @@ If you want to run other applications, such as the JS REPL or the WebAssembly RE
 Ladybird will be built with one of the following browser frontends, depending on the platform:
 * [AppKit](https://developer.apple.com/documentation/appkit?language=objc) - The native UI on macOS.
 * [Qt](https://doc.qt.io/qt-6/) - The UI used on all other platforms.
+* [GTK 4](https://docs.gtk.org/gtk4/) - An alternative UI on Linux (experimental).
 * [Android UI](https://developer.android.com/develop/ui) - The native UI on Android.
 
-The Qt UI is available on platforms where it is not the default as well (except on Android).
-You can pick the UI using the `LADYBIRD_GUI_FRAMEWORK` option, for example to enable the Qt UI:
+The Qt and GTK UIs are available on Linux. You can pick the UI using the `LADYBIRD_GUI_FRAMEWORK` option:
 
 ```bash
 # From /path/to/ladybird
 cmake --preset Release -DLADYBIRD_GUI_FRAMEWORK=Qt
+cmake --preset Release -DLADYBIRD_GUI_FRAMEWORK=Gtk
+```
+
+#### Additional prerequisites for the GTK UI
+
+Building with `LADYBIRD_GUI_FRAMEWORK=Gtk` requires additional system packages, as some vcpkg
+dependencies (e.g. gettext) need to be rebuilt from source:
+
+**Debian/Ubuntu:**
+```bash
+sudo apt install bison libxkbcommon-dev
+```
+
+**Arch Linux/Manjaro:**
+```bash
+sudo pacman -S bison
+```
+
+**Fedora:**
+```bash
+sudo dnf install bison
 ```
 
 ### Build error messages you may encounter

Documentation/GtkFrontend.md

@@ -0,0 +1,105 @@
+# GTK Frontend
+
+GTK4/libadwaita frontend for Ladybird.
+
+## Style Guide
+
+Follow the project [Coding Style](CodingStyle.md). Prefer C++ over C idioms wherever possible. Below are GTK-specific rules.
+
+### General
+- Do not duplicate functionality already in LibWebView — the frontend is a thin shell
+- Follow [GNOME HIG](https://developer.gnome.org/hig/) — prefer libadwaita widgets (`AdwTabView`, `AdwHeaderBar`, `AdwAlertDialog`, `AdwToast`, etc.) over custom implementations
+- Prefer C++ types and patterns over GLib/GObject equivalents
+- Use D-Bus via `GApplication` for single-instance and IPC — not custom socket/file mechanisms
+- Prefer file-static functions over anonymous namespaces for internal helpers
+- Namespace: `Ladybird::` for C++ classes, `LadybirdWidgets::` for GObject widget helpers
+
+### GObject
+- Only register GTypes (`G_DEFINE_FINAL_TYPE`) for widgets used in GtkBuilder templates
+- No `G_DECLARE_FINAL_TYPE` — define structs and `GType` functions manually to avoid reserved `_TypeName` names
+- Use `GtkBuilder` and `.ui` resource files for declarative layout where practical
+
+### Signals and Callbacks
+- No `bind_template_callback` — connect signals in C++ with `g_signal_connect_swapped`
+- No `g_signal_new` — use `Function<>` callbacks instead of custom GObject signals
+- Prefer `g_signal_connect_swapped` over `g_signal_connect` to avoid `static_cast<Foo*>(user_data)`
+- For buttons in popover menus, prefer `action-name` properties over click signal handlers
+
+### Memory Management
+- No manual `g_object_unref` — use `GObjectPtr<T>` (see `UI/Gtk/GLibPtr.h`)
+- No manual `g_free` — use `g_autofree`
+
+```cpp
+// Local scope — auto-unrefs when out of scope
+GObjectPtr builder { gtk_builder_new_from_resource("/path/to/file.ui") };
+
+// Class member
+GObjectPtr<GdkTexture> m_texture;
+m_texture = GObjectPtr<GdkTexture> { gdk_memory_texture_builder_build(builder) };
+
+// g_autofree for glib-allocated strings
+g_autofree char* path = g_file_get_path(file);
+```
+
+## Test Checklist
+
+### Navigation
+- [ ] Load URL from location bar
+- [ ] Back/forward buttons
+- [ ] Reload (Ctrl+R, F5)
+- [ ] New tab (Ctrl+T)
+- [ ] Close tab (Ctrl+W)
+- [ ] New window (Ctrl+N)
+- [ ] Tab switching
+- [ ] Child tab creation (target=_blank)
+
+### Input
+- [ ] Keyboard input in web content
+- [ ] Mouse clicks, drag, selection
+- [ ] Touchpad smooth scrolling
+- [ ] Mouse wheel discrete scrolling
+- [ ] Ctrl+scroll zoom
+- [ ] Right-click context menus (page, link, image, media)
+
+### UI
+- [ ] Location bar URL display with domain highlighting
+- [ ] Location bar autocomplete
+- [ ] Security icon (https/insecure)
+- [ ] Find in page (Ctrl+F)
+- [ ] Fullscreen (F11)
+- [ ] Zoom in/out/reset (Ctrl+=/-/0)
+- [ ] Tab loading indicator
+- [ ] Cursor changes (pointer, text, resize, etc.)
+- [ ] Tooltips on hover
+- [ ] Dark/light theme follows system
+
+### Dialogs
+- [ ] JavaScript alert
+- [ ] JavaScript confirm
+- [ ] JavaScript prompt
+- [ ] Color picker
+- [ ] File picker (single and multiple)
+- [ ] Select dropdown
+- [ ] Download save dialog
+- [ ] Download confirmation toast
+- [ ] Error dialog
+
+### Window Management
+- [ ] Minimize/maximize/close
+- [ ] Resize
+- [ ] Fullscreen enter/exit
+- [ ] D-Bus single instance (second launch opens tab in existing window)
+
+### Internal Pages
+- [ ] about:version
+- [ ] about:settings
+- [ ] about:processes (Task Manager)
+
+### Clipboard
+- [ ] Copy (Ctrl+C)
+- [ ] Paste (Ctrl+V)
+- [ ] Select all (Ctrl+A)
+
+### DevTools
+- [ ] Enable/disable via banner
+- [ ] Toggle (Ctrl+Shift+I, F12)