Missing window decorations on GNOME / Wayland

[khromov]

A GPUI window has no title bar, and no close / minimise / maximise buttons, on GNOME —
which is Ubuntu's default session. The same binary is decorated normally on sway, labwc
and KDE, so the app looks broken only for GNOME users.

Component	gpuix-native 0.7.0 (packages/native), gpui_linux Wayland backend
Affects	GNOME / Mutter (any version — Mutter implements no server-side decorations)
Verified on	Mutter 46.2, Ubuntu 24.04, wgpu/Vulkan via lavapipe

Actual — GNOME / Mutter 46.2

No title bar under Mutter

Expected — the same build under labwc

Title bar and buttons under labwc

Same component, same binary, same window size. The only variable is the compositor.

Cause

gpuix never tells the compositor which decoration mode it wants, so the compositor's
default decides.

1. A window starts client-side decorated — gpui_linux/src/linux/wayland/window.rs:612,
   decorations: WindowDecorations::Client.
2. gpui creates a zxdg_toplevel_decoration_v1 when the manager exists
   (window.rs:279) but never calls set_mode on it.
3. gpuix never calls request_decorations either. Its WindowOptions.titlebar_transparent
   feeds gpui::TitlebarOptions, which is the macOS titlebar and does nothing here.

With no set_mode, the protocol leaves the choice to the compositor:

$ wayland-info | grep -i decoration      # labwc
interface: 'org_kde_kwin_server_decoration_manager', version: 1, name: 16
interface: 'zxdg_decoration_manager_v1',             version: 1, name: 17

$ wayland-info | grep -i decoration      # mutter 46.2
(no output — 26 globals, none of them a decoration manager)

sway / labwc / KDE advertise the manager and default to server-side, so they decorate the
window. Mutter advertises none, so decoration stays None, the window stays
WindowDecorations::Client, and since a gpuix view paints only the application's element
tree, nothing draws the chrome at all.

Reproduce

From gpuix-svelte, which packages both sessions:

npm run linux:build
npm run linux:demo -- counter --gnome     # Mutter 46.2 — no title bar
npm run linux:demo -- counter --desktop   # labwc      — title bar with buttons

Both open the window at http://localhost:6080.

Suggested fix

GNOME mandates client-side decorations; there is no server-side mode to switch on there.
So the application has to draw the bar, and gpuix has to make that possible and consistent:

1. Expose requestDecorations(mode). PlatformWindow::request_decorations already
   maps to set_mode (window.rs:2062, 2228). Calling it with Client also suppresses
   the server-side bar on sway / labwc / KDE — without that, an app that draws its own bar
   gets two.
2. Report the negotiated mode back to the app. The compositor's reply arrives at
   handle_toplevel_decoration_event (window.rs:1135) and need not match the request, so
   the app should draw its bar only when the result is ClientSide.
3. Expose the window controls a title bar needs — gpui has minimize(), zoom() and
   start_window_move() (gpui/src/platform.rs:845-907); none are bound in
   @gpuix/native's index.d.ts. Without start_window_move a custom bar cannot drag the
   window, since on Wayland only the compositor may move it.

Worth noting for whoever implements it: is_transparent() returns true whenever
decorations are client-side (window.rs:635), so under CSD the window is transparent and
the app must paint its own background and corners. client_inset / getWindowInsets is
the related plumbing.