Cover every output from a background shell: the session-lock secondary machinery generalizes to layer surfaces

The runtime already covered outputs beyond the first, but only for `ShellMode::SessionLock`: `locked` created one `ext-session-lock` surface per output, tracked in `lock_extras`, rendered from `App::lock_secondary_view`, excluded from pointer/touch routing and repainted through the shared invalidation path. A background shell had nothing equivalent, so on a multi-monitor session its wallpaper simply did not exist on the extended screens — the compositor had to fake one by cover-scaling the primary's background, wrong scale and wrong crop included.
The machinery is shared rather than duplicated. The state and the hook lose their lock-specific names — `lock_extras` becomes `secondaries`, `main_lock_output` becomes `main_output`, `App::lock_secondary_view` becomes `App::secondary_view` — and everything downstream of creation (the draw loop in frame.rs, invalidation, per-surface scale handling, the input exclusion in pointer/touch, redraw scheduling in run.rs) is the same single copy serving both kinds.
What is genuinely new is the second creation site: in `new_output`, when the app's shell mode is `Layer( Background )`, every output beyond the main surface's gets its own background layer surface — anchored to all four edges with size delegated to the compositor, exclusive zone 0, no keyboard interactivity — bound to that output at creation and torn down in `output_destroyed`. The main surface's output is now recorded when it materializes so the runtime knows which output not to cover.
Layer-shell events learn about secondaries too, and the `configure` branch is not optional: secondaries live outside the focus map, so their configure used to fall into the `None` arm and resize the main surface. `closed` on a secondary now just drops it instead of asking the app to exit.
`secondary_view` keeps its default of `None` (surface filled with `background_color`), so nothing changes for apps that do not opt in; the session-lock behaviour is unchanged aside from the renames.

src/app.rs

@@ -794,15 +794,16 @@ pub trait App: 'static
 	/// returning `true` from [`requested_exit`](Self::requested_exit).
 	fn shell_mode( &self ) -> ShellMode { ShellMode::Window }
 
-	/// Content for the lock surfaces the runtime creates on every output
-	/// beyond the first while in [`ShellMode::SessionLock`]. The compositor
-	/// blanks any output without a lock surface, so the runtime covers each
-	/// one; this view is what gets painted there (typically the same
-	/// wallpaper as the main surface, without the interactive form).
+	/// Content for the surfaces the runtime creates on every output beyond
+	/// the first. In [`ShellMode::SessionLock`] these are lock surfaces —
+	/// the compositor blanks any output without one, so each is covered. In
+	/// [`ShellMode::Layer`] on [`Layer::Background`] they are background
+	/// layer surfaces, so a shell's wallpaper reaches extended outputs
+	/// (typically without the grid or any chrome). They take no input.
 	/// `width` / `height` are that output's surface size in physical pixels.
 	/// Default `None`: the surface is filled with
 	/// [`background_color`](Self::background_color).
-	fn lock_secondary_view( &self, _width: u32, _height: u32 ) -> Option<Element<Self::Message>>
+	fn secondary_view( &self, _width: u32, _height: u32 ) -> Option<Element<Self::Message>>
 	{
 		None
 	}

src/event_loop/app_data.rs

@@ -219,15 +219,17 @@ pub struct AppData<A: App>
 	/// Populated and reconciled by the run loop from [`App::overlays`]. Always
 	/// empty until overlay diffing is wired up.
 	pub overlays:            HashMap<OverlayId, SurfaceState<A::Message>>,
-	/// Session-lock surfaces covering every output beyond the first, paired
-	/// with the output each one belongs to. The compositor blanks any output
-	/// without a lock surface, so one is created per output; these render
-	/// [`App::lock_secondary_view`] and take no input (the input handlers
-	/// find no focus for them, and keyboard falls through to `Main`).
-	pub lock_extras:         Vec<( WlOutput, SurfaceState<A::Message> )>,
-	/// Output the main lock surface was created on, so a later `new_output`
-	/// for the same output does not add a duplicate lock surface.
-	pub main_lock_output:    Option<WlOutput>,
+	/// Surfaces covering every output beyond the first, paired with the
+	/// output each one belongs to: session-lock surfaces while locked (the
+	/// compositor blanks any output without one), background layer surfaces
+	/// for a [`crate::Layer::Background`] shell (the wallpaper reaches
+	/// extended outputs). These render [`App::secondary_view`] and take no
+	/// input (the input handlers find no focus for them, and keyboard falls
+	/// through to `Main`).
+	pub secondaries:         Vec<( WlOutput, SurfaceState<A::Message> )>,
+	/// Output the main surface was created on, so a later `new_output` for
+	/// the same output does not add a duplicate secondary surface.
+	pub main_output:         Option<WlOutput>,
 	/// Input-transparent child surfaces keyed by their stable
 	/// [`SubsurfaceId`]. Reconciled each frame from [`App::subsurfaces`].
 	pub subsurfaces:         HashMap<SubsurfaceId, SubsurfaceSlot>,
@@ -295,14 +297,14 @@ impl<A: App> AppData<A>
 		None
 	}
 
-	/// Whether `wl` is one of the secondary session-lock surfaces. Pointer and
+	/// Whether `wl` is one of the secondary per-output surfaces. Pointer and
 	/// touch events landing on them must be dropped rather than falling back
 	/// to `Main` — their coordinates would hit-test the main surface's
 	/// widgets. Keyboard keeps the `Main` fallback so typing works wherever
-	/// the compositor puts the lock focus.
-	pub( crate ) fn is_lock_extra_surface( &self, wl: &WlSurface ) -> bool
+	/// the compositor puts the focus.
+	pub( crate ) fn is_secondary_surface( &self, wl: &WlSurface ) -> bool
 	{
-		self.lock_extras.iter().any( |( _, ss )| ss.surface.try_wl_surface() == Some( wl ) )
+		self.secondaries.iter().any( |( _, ss )| ss.surface.try_wl_surface() == Some( wl ) )
 	}
 
 	/// Borrow the [`SurfaceState`] identified by `focus`. Panics if `focus`

src/event_loop/frame.rs

@@ -71,11 +71,11 @@ pub( crate ) fn draw_frame<A: App>( data: &mut AppData<A> ) -> bool
 		}
 	}
 
-	for ( _, ss ) in data.lock_extras.iter_mut()
+	for ( _, ss ) in data.secondaries.iter_mut()
 	{
 		if !ss.configured || !ss.needs_redraw || ss.frame_pending { continue; }
 		let scale = ss.scale_factor.max( 1 ) as u32;
-		let view  = data.app.lock_secondary_view( ss.width * scale, ss.height * scale )
+		let view  = data.app.secondary_view( ss.width * scale, ss.height * scale )
 			.unwrap_or_else( || crate::spacer().into() );
 		// No frame callback: these surfaces are static between invalidations,
 		// so there is no per-frame pacing to keep — and a callback that never

src/event_loop/handlers.rs

@@ -92,9 +92,9 @@ impl<A: App> CompositorHandler for AppData<A>
 		if new_factor <= 0 { return; }
 		let Some( focus ) = self.focus_for_surface( surface ) else
 		{
-			// Lock extras live outside the focus map.
+			// Secondary per-output surfaces live outside the focus map.
 			let shm = &self.shm;
-			if let Some( ( _, ss ) ) = self.lock_extras.iter_mut()
+			if let Some( ( _, ss ) ) = self.secondaries.iter_mut()
 				.find( |( _, ss )| ss.surface.try_wl_surface() == Some( surface ) )
 			{
 				apply_surface_scale( ss, shm, surface, new_factor );
@@ -165,6 +165,11 @@ impl<A: App> LayerShellHandler for AppData<A>
 		layer: &LayerSurface,
 	)
 	{
+		if self.is_secondary_surface( layer.wl_surface() )
+		{
+			self.secondaries.retain( |( _, ss )| ss.surface.try_wl_surface() != Some( layer.wl_surface() ) );
+			return;
+		}
 		match self.focus_for_surface( layer.wl_surface() )
 		{
 			Some( super::SurfaceFocus::Main ) | None =>
@@ -200,6 +205,17 @@ impl<A: App> LayerShellHandler for AppData<A>
 	{
 		let ( w, h ) = configure.new_size;
 		let ( w, h ) = ( w.max( 1 ), h.max( 1 ) );
+		// Secondaries live outside the focus map; without this branch their
+		// configure would fall into the `None` arm and resize the main
+		// surface instead.
+		for ( _, ss ) in self.secondaries.iter_mut()
+		{
+			if ss.surface.try_wl_surface() == Some( layer.wl_surface() )
+			{
+				ss.on_configure( &self.shm, self.egl_context.as_ref(), w, h );
+				return;
+			}
+		}
 		match self.focus_for_surface( layer.wl_surface() )
 		{
 			Some( super::SurfaceFocus::Main ) | None =>
@@ -290,23 +306,56 @@ impl<A: App> OutputHandler for AppData<A>
 		// Same for any overlays that were created before an output existed.
 		if let Some( ref layer_shell ) = self.layer_shell
 		{
-			self.main.surface.materialize( &self.compositor_state, layer_shell, qh, &output );
+			if self.main.surface.materialize( &self.compositor_state, layer_shell, qh, &output )
+			{
+				self.main_output = Some( output.clone() );
+			}
 			for ss in self.overlays.values_mut()
 			{
 				ss.surface.materialize( &self.compositor_state, layer_shell, qh, &output );
 			}
+			// A background shell covers every output: each one beyond the
+			// main's gets its own background layer surface, painted with
+			// `App::secondary_view` — the wallpaper without any chrome.
+			if self.app.shell_mode() == crate::app::ShellMode::Layer( crate::app::Layer::Background )
+				&& self.main_output.is_some()
+				&& self.main_output.as_ref() != Some( &output )
+				&& !self.secondaries.iter().any( |( o, _ )| o == &output )
+			{
+				use smithay_client_toolkit::shell::wlr_layer;
+				let surface       = self.compositor_state.create_surface( qh );
+				let layer_surface = layer_shell.create_layer_surface(
+					qh,
+					surface,
+					wlr_layer::Layer::Background,
+					Some( "ltk-secondary" ),
+					Some( &output ),
+				);
+				layer_surface.set_anchor(
+					wlr_layer::Anchor::TOP | wlr_layer::Anchor::BOTTOM
+						| wlr_layer::Anchor::LEFT | wlr_layer::Anchor::RIGHT,
+				);
+				layer_surface.set_exclusive_zone( 0 );
+				layer_surface.set_keyboard_interactivity( wlr_layer::KeyboardInteractivity::None );
+				layer_surface.set_size( 0, 0 );
+				layer_surface.commit();
+				self.secondaries.push( (
+					output.clone(),
+					super::SurfaceState::new( super::SurfaceKind::Layer( layer_surface ), 0.0, String::new() ),
+				) );
+			}
 		}
 		// An output hotplugged while the session is locked would be blanked by
 		// the compositor until it gets a lock surface of its own.
 		if let Some( ref lock ) = self.session_lock
 		{
 			if matches!( self.main.surface, super::SurfaceKind::Lock( .. ) )
-				&& self.main_lock_output.as_ref() != Some( &output )
-				&& !self.lock_extras.iter().any( |( o, _ )| o == &output )
+				&& self.main_output.as_ref() != Some( &output )
+				&& !self.secondaries.iter().any( |( o, _ )| o == &output )
 			{
 				let surface      = self.compositor_state.create_surface( qh );
 				let lock_surface = lock.create_lock_surface( surface, &output, qh );
-				self.lock_extras.push( (
+				self.secondaries.push( (
 					output,
 					super::SurfaceState::new( super::SurfaceKind::Lock( lock_surface ), 0.0, String::new() ),
 				) );
@@ -318,10 +367,10 @@ impl<A: App> OutputHandler for AppData<A>
 
 	fn output_destroyed( &mut self, _: &Connection, _: &QueueHandle<Self>, output: WlOutput )
 	{
-		self.lock_extras.retain( |( o, _ )| o != &output );
-		if self.main_lock_output.as_ref() == Some( &output )
+		self.secondaries.retain( |( o, _ )| o != &output );
+		if self.main_output.as_ref() == Some( &output )
 		{
-			self.main_lock_output = None;
+			self.main_output = None;
 		}
 	}
 }
@@ -484,7 +533,7 @@ impl<A: App> SessionLockHandler for AppData<A>
 	{
 		// Every output needs its own lock surface — the compositor blanks any
 		// output left without one. The first hosts the app's main surface;
-		// the rest render `App::lock_secondary_view`.
+		// the rest render `App::secondary_view`.
 		let outputs: Vec<WlOutput> = self.output_state.outputs().collect();
 		let mut outputs = outputs.into_iter();
 		if let Some( output ) = outputs.next()
@@ -492,13 +541,13 @@ impl<A: App> SessionLockHandler for AppData<A>
 			let surface      = self.compositor_state.create_surface( qh );
 			let lock_surface = session_lock.create_lock_surface( surface, &output, qh );
 			self.main.surface = super::SurfaceKind::Lock( lock_surface );
-			self.main_lock_output = Some( output );
+			self.main_output  = Some( output );
 		}
 		for output in outputs
 		{
 			let surface      = self.compositor_state.create_surface( qh );
 			let lock_surface = session_lock.create_lock_surface( surface, &output, qh );
-			self.lock_extras.push( (
+			self.secondaries.push( (
 				output,
 				super::SurfaceState::new( super::SurfaceKind::Lock( lock_surface ), 0.0, String::new() ),
 			) );
@@ -529,7 +578,7 @@ impl<A: App> SessionLockHandler for AppData<A>
 			self.on_configure( w, h );
 			return;
 		}
-		for ( _, ss ) in self.lock_extras.iter_mut()
+		for ( _, ss ) in self.secondaries.iter_mut()
 		{
 			if ss.surface.try_wl_surface() == Some( wl )
 			{

src/event_loop/invalidation.rs

@@ -27,7 +27,7 @@ pub( super ) fn apply_invalidation<A: App>( data: &mut AppData<A>, scope: Invali
 			{
 				ss.request_redraw();
 			}
-			for ( _, ss ) in data.lock_extras.iter_mut()
+			for ( _, ss ) in data.secondaries.iter_mut()
 			{
 				ss.request_redraw();
 			}
@@ -44,7 +44,7 @@ pub( super ) fn apply_invalidation<A: App>( data: &mut AppData<A>, scope: Invali
 						data.main.request_redraw();
 						// Lock extras mirror the main surface's backdrop, so
 						// they follow its invalidations.
-						for ( _, ss ) in data.lock_extras.iter_mut()
+						for ( _, ss ) in data.secondaries.iter_mut()
 						{
 							ss.request_redraw();
 						}

src/event_loop/run.rs

@@ -320,8 +320,8 @@ pub( crate ) fn try_run<A: App>( app: A ) -> Result<(), RunError>
 		pending_size_hint_unpin,
 		main:                SurfaceState::<A::Message>::new( surface_kind, titlebar_height, titlebar_title ),
 		overlays:            std::collections::HashMap::new(),
-		lock_extras:         Vec::new(),
-		main_lock_output:    None,
+		secondaries:         Vec::new(),
+		main_output:         None,
 		subsurfaces:         std::collections::HashMap::new(),
 		subsurface_gles_canvas: None,
 		pointer_focus:       SurfaceFocus::Main,
@@ -600,7 +600,7 @@ pub( crate ) fn try_run<A: App>( app: A ) -> Result<(), RunError>
 		// surface qualifies we just loop back to `dispatch(None)` and sleep.
 		let any_drawable = ( data.main.configured && data.main.needs_redraw && !data.main.frame_pending )
 			|| data.overlays.values().any( |ss| ss.configured && ss.needs_redraw && !ss.frame_pending )
-			|| data.lock_extras.iter().any( |( _, ss )| ss.configured && ss.needs_redraw && !ss.frame_pending );
+			|| data.secondaries.iter().any( |( _, ss )| ss.configured && ss.needs_redraw && !ss.frame_pending );
 		if any_drawable
 		{
 			// Rebuild while motion is in progress and on the first frame

src/input/pointer/mod.rs

@@ -51,7 +51,7 @@ impl<A: App> PointerHandler for AppData<A>
 	{
 		for event in events
 		{
-			if self.is_lock_extra_surface( &event.surface ) { continue; }
+			if self.is_secondary_surface( &event.surface ) { continue; }
 			let focus = self.focus_for_surface( &event.surface )
 				.unwrap_or( SurfaceFocus::Main );
 			self.pointer_focus = focus;

src/input/touch/mod.rs

@@ -60,7 +60,7 @@ impl<A: App> TouchHandler for AppData<A>
 	)
 	{
 		self.last_input_serial = serial;
-		if self.is_lock_extra_surface( &surface ) { return; }
+		if self.is_secondary_surface( &surface ) { return; }
 		let focus = self.focus_for_surface( &surface ).unwrap_or( SurfaceFocus::Main );
 		self.touch_focus.insert( id, focus );
 		let pos = self.surface( focus ).to_physical( position.0, position.1 );