Fix Widget changed layer_id panic when undocking tabs (debug builds)

src/widgets/dock_area/show/leaf.rs

@@ -20,6 +20,10 @@ use crate::{
     DockArea, Node, NodeIndex, Style, SurfaceIndex, TabAddAlign, TabIndex, TabStyle, TabViewer,
 };
 
+fn tab_body_id(dock_area_id: Id, path: NodePath, tab_id: Id) -> Id {
+    dock_area_id.with((path.surface, "surface")).with(tab_id)
+}
+
 impl<Tab> DockArea<'_, Tab> {
     pub(super) fn show_leaf(
         &mut self,
@@ -1218,7 +1222,7 @@ impl<Tab> DockArea<'_, Tab> {
                 // We are forced to use `Ui::new` because other methods (eg: push_id) always mix
                 // the provided id with their own which would cause tabs to change id when moved
                 // from node to node.
-                let id = self.id.with(tab_viewer.id(tab));
+                let id = tab_body_id(self.id, path, tab_viewer.id(tab));
                 ui.ctx().check_for_id_clash(id, body_rect, "a tab with id");
                 let ui = &mut Ui::new(
                     ui.ctx().clone(),
@@ -1313,3 +1317,31 @@ impl<Tab> DockArea<'_, Tab> {
         }
     }
 }
+
+#[cfg(test)]
+mod tests {
+    use super::tab_body_id;
+    use crate::{NodeIndex, NodePath, SurfaceIndex};
+    use egui::Id;
+
+    #[test]
+    fn tab_body_ids_differ_between_surfaces() {
+        let dock_area_id = Id::new("dock-area");
+        let tab_id = Id::new("same-tab");
+        let node = NodeIndex::root();
+
+        let main = NodePath {
+            surface: SurfaceIndex::main(),
+            node,
+        };
+        let detached = NodePath {
+            surface: SurfaceIndex(1),
+            node,
+        };
+
+        assert_ne!(
+            tab_body_id(dock_area_id, main, tab_id),
+            tab_body_id(dock_area_id, detached, tab_id)
+        );
+    }
+}