Fix #2646: keep undocked shared-dock panels opaque

Floating groups on the canvas now always get a panel-colour backdrop from layout geometry when egui has not registered the area yet, and panel bodies paint their own fill so empty regions do not show through.

Co-authored-by: Cursor <cursoragent@cursor.com>

crates/ui-egui/src/panel_docking.rs

@@ -504,6 +504,11 @@ struct PanelContent<'a> {
 
 impl craft_ui::docking::DockContent<String> for PanelContent<'_> {
     fn body(&mut self, ui: &mut egui::Ui, panel: &String) {
+        let t = crate::theme::Tokens::get(ui.ctx());
+        let back = ui.max_rect();
+        if back.is_positive() {
+            ui.painter().rect_filled(back, 0.0, t.panel);
+        }
         let width = ui.available_width();
         let height = (ui.available_height() - 20.0).max(0.0);
         if crate::state::DockTab::from_id(panel).is_some() {
@@ -637,10 +642,13 @@ pub(crate) fn show(app: &mut VectorcraftApp, ui: &mut egui::Ui) -> bool {
     let fills: Vec<_> = display
         .floating
         .iter()
-        .filter_map(|group| group.panels.first())
-        .map(|first| {
+        .filter_map(|group| {
+            let first = group.panels.first()?;
             let layer = egui::LayerId::new(egui::Order::Middle, area.with(("floating", first)));
-            (layer, ui.ctx().layer_painter(layer).add(egui::Shape::Noop))
+            let [x, y, w, h] = group.rect;
+            let layout_rect = egui::Rect::from_min_size(egui::pos2(x, y), egui::vec2(w, h));
+            let fill = ui.ctx().layer_painter(layer).add(egui::Shape::Noop);
+            Some((layer, layout_rect, fill))
         })
         .collect();
     let mut draw = |ui: &mut egui::Ui| {
@@ -787,8 +795,13 @@ pub(crate) fn show(app: &mut VectorcraftApp, ui: &mut egui::Ui) -> bool {
             ui.ctx().data_mut(|data| data.insert_temp(crate::floating::group_rect_id(first), rect));
         }
     }
-    for (layer, fill) in fills {
-        if let Some(rect) = ui.ctx().memory(|memory| memory.area_rect(layer.id)) {
+    for (layer, layout_rect, fill) in fills {
+        let rect = ui
+            .ctx()
+            .memory(|memory| memory.area_rect(layer.id))
+            .filter(|rect| rect.is_positive())
+            .unwrap_or(layout_rect);
+        if rect.is_positive() {
             ui.ctx().layer_painter(layer).set(fill, egui::Shape::rect_filled(rect, 0.0, t.panel));
         }
     }

crates/ui-egui/src/panel_docking_tests.rs

@@ -616,7 +616,7 @@ fn the_window_menu_item_hides_a_shown_panel_and_shows_it_again() {
 }
 
 /// A floating group lies on the canvas, not on the dock's card, so its tab strip and body paint
-/// their own opaque background (#2421).
+/// their own opaque background (#2421, #2646).
 #[test]
 fn floating_group_paints_an_opaque_background() {
     let mut app = app();
@@ -641,6 +641,27 @@ fn floating_group_paints_an_opaque_background() {
     assert!(opaque_at(pos2(690.0, 610.0)), "body is transparent");
 }
 
+/// Undocking a tabbed dock panel leaves an opaque backdrop even in the empty part of its body (#2646).
+#[test]
+fn undocked_layers_panel_body_is_opaque() {
+    let mut app = app();
+    app.run("window.panel.float", json!({"panel":"layers","x":300,"y":160,"width":300,"height":420})).unwrap();
+    let h = harness(app, vec2(1280.0, 800.0));
+    fn topmost(shape: &egui::Shape, p: Pos2, found: &mut Option<egui::Color32>) {
+        match shape {
+            egui::Shape::Rect(r) if r.rect.contains(p) && r.rect.width() >= 80.0 && r.stroke.is_empty() && r.fill.a() > 0 => *found = Some(r.fill),
+            egui::Shape::Vec(v) => v.iter().for_each(|s| topmost(s, p, found)),
+            _ => {}
+        }
+    }
+    let opaque_at = |p: Pos2| {
+        let mut found = None;
+        h.output().shapes.iter().for_each(|s| topmost(&s.shape, p, &mut found));
+        found.is_some_and(|c| c.a() == 255)
+    };
+    assert!(opaque_at(pos2(580.0, 520.0)), "empty layers body shows the canvas");
+}
+
 /// Library panels (#2603) float in the shared layout titled by their libraries' names, and their
 /// menu's Persistent item keeps them for the next launch.
 #[test]