shell: Restore fullscreen to single-element stack

If a surface was previously a single-element stack, restore it normally
using the floating/tiling restore data, but map as a single element
stack instead of a plain window.

src/shell/mod.rs

@@ -12,7 +12,10 @@ use std::{
 use wayland_backend::server::ClientId;
 
 use crate::{
-    shell::{focus::FocusTarget, grabs::fullscreen_items, layout::tiling::PlaceholderType},
+    shell::{
+        element::CosmicStack, focus::FocusTarget, grabs::fullscreen_items,
+        layout::tiling::PlaceholderType,
+    },
     wayland::{
         handlers::data_device::{self, get_dnd_icon},
         protocols::workspace::{State as WState, WorkspaceCapabilities},
@@ -2607,12 +2610,21 @@ impl Shell {
             }
         }
 
-        let window = CosmicMapped::from(CosmicWindow::new(
-            surface,
-            loop_handle.clone(),
-            self.theme.clone(),
-            self.appearance_conf,
-        ));
+        let window = if state.as_ref().is_some_and(|s| s.was_stack()) {
+            CosmicMapped::from(CosmicStack::new(
+                std::iter::once(surface),
+                loop_handle.clone(),
+                self.theme.clone(),
+                self.appearance_conf,
+            ))
+        } else {
+            CosmicMapped::from(CosmicWindow::new(
+                surface,
+                loop_handle.clone(),
+                self.theme.clone(),
+                self.appearance_conf,
+            ))
+        };
 
         if let Some(FullscreenRestoreState::Sticky { output, state, .. }) = &state {
             let output = output
@@ -3267,6 +3279,7 @@ impl Shell {
                         state: None,
                         was_maximized: previous.was_maximized(),
                     },
+                    was_stack: previous.was_stack(),
                 };
             } else if let FullscreenRestoreState::Tiling { workspace, .. }
             | FullscreenRestoreState::Floating { workspace, .. } = previous
@@ -4808,6 +4821,7 @@ impl Shell {
             let mut from = set.sticky_layer.element_geometry(&mapped).unwrap();
             let mut was_maximized = false;
             let mut restore_state = None;
+            let was_stack = mapped.is_stack();
             window = if let Some(stack) = mapped.stack_ref()
                 && stack.len() > 1
             {
@@ -4859,6 +4873,7 @@ impl Shell {
                         was_maximized,
                         was_snapped: None,
                     },
+                    was_stack,
                 })),
                 Some(from),
             );
@@ -4893,12 +4908,14 @@ impl Shell {
                         Some(FullscreenRestoreState::Floating {
                             workspace: handle,
                             state: floating_state,
+                            was_stack: mapped.is_stack(),
                         })
                     }
                     WorkspaceRestoreData::Tiling(tiling_state) => {
                         Some(FullscreenRestoreState::Tiling {
                             workspace: handle,
                             state: tiling_state,
+                            was_stack: mapped.is_stack(),
                         })
                     }
                     WorkspaceRestoreData::Stack(stack_state) => {

src/shell/workspace.rs

@@ -254,14 +254,17 @@ pub enum FullscreenRestoreState {
     Tiling {
         workspace: WorkspaceHandle,
         state: TilingRestoreData,
+        was_stack: bool,
     },
     Floating {
         workspace: WorkspaceHandle,
         state: FloatingRestoreData,
+        was_stack: bool,
     },
     Sticky {
         output: WeakOutput,
         state: FloatingRestoreData,
+        was_stack: bool,
     },
     Stack {
         state: StackRestoreData,
@@ -277,6 +280,17 @@ impl FullscreenRestoreState {
             FullscreenRestoreState::Stack { .. } => false,
         }
     }
+
+    // Surface was previously a single-window stack
+    pub fn was_stack(&self) -> bool {
+        match self {
+            FullscreenRestoreState::Floating { was_stack, .. }
+            | FullscreenRestoreState::Sticky { was_stack, .. } => *was_stack,
+            FullscreenRestoreState::Tiling { was_stack, .. } => *was_stack,
+            // Stack wasn't removed; surface was removed from the stack
+            FullscreenRestoreState::Stack { .. } => false,
+        }
+    }
 }
 
 #[derive(Debug, Clone)]