event_loop: drop the built-in titlebar when the compositor decorates

ltk already asked for server-side decorations through SCTK (WindowDecorations::RequestServer on window creation) but painted its 36 px titlebar unconditionally, so under forge an xdg window carried two stacked bars: forge's SSD on top of ltk's own. The WindowHandler configure now honours the negotiated decoration_mode: Server zeroes titlebar_height, Client — which is also what SCTK reports when the compositor lacks xdg-decoration, e.g. GNOME — restores it from the new titlebar_base field. Every titlebar consumer (draw, close-button hit test, drag-move) already guards on the height being positive, so the suppressed bar costs nothing and the fallback path is unchanged.

src/event_loop/handlers.rs

@@ -19,7 +19,7 @@ use smithay_client_toolkit::
 		wlr_layer::{ LayerShellHandler, LayerSurface, LayerSurfaceConfigure },
 		xdg::XdgSurface,
 		xdg::popup::{ Popup, PopupConfigure, PopupHandler },
-		xdg::window::{ Window, WindowConfigure, WindowHandler },
+		xdg::window::{ DecorationMode, Window, WindowConfigure, WindowHandler },
 	},
 	shm::{ Shm, ShmHandler },
 	session_lock::{ SessionLock, SessionLockHandler, SessionLockSurface, SessionLockSurfaceConfigure },
@@ -222,6 +222,14 @@ impl<A: App> WindowHandler for AppData<A>
 			window.set_max_size( None );
 			self.pending_size_hint_unpin = false;
 		}
+		let tb = if configure.decoration_mode == DecorationMode::Server { 0.0 }
+			else { self.main.titlebar_base };
+		if self.main.titlebar_height != tb
+		{
+			self.main.titlebar_height = tb;
+			self.main.content_dirty   = true;
+			self.view_dirty           = true;
+		}
 		let ( hint_w, hint_h ) = self.app.window_size_hint().unwrap_or( ( 800, 600 ) );
 		let w = configure.new_size.0.map( |v| v.get() ).unwrap_or( hint_w );
 		let h = configure.new_size.1.map( |v| v.get() ).unwrap_or( hint_h );