Fix #649: a panel dropped on the dock lights up where it goes

Panels other than Properties, Layers and Libraries live in the dock's icon
column, not as tabs. Dragging one onto the dock lit up the whole dock, tabs
included, and the drop put it back as an icon, so it seemed to vanish. The
drop zone now lights up the icon column for such panels (the whole dock still
for the tabbed ones), and the dropped panel pops out of its icon.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

crates/ui-egui/src/dock.rs

@@ -151,6 +151,7 @@ pub fn show(app: &mut VectorcraftApp, ui: &mut Ui) {
     ui.ctx().data_mut(|d| {
         d.insert_temp(column_left_id(), column.left());
         d.insert_temp(floating::dock_rect_id(), dock);
+        d.insert_temp(floating::icons_rect_id(), column);
     });
 }
 

crates/ui-egui/src/floating.rs

@@ -70,6 +70,12 @@ pub(crate) fn dock_rect_id() -> Id {
 }
 
 /// The strip along the window's left edge where the Tools panel docks, as last laid out.
+/// The icon column's bounds (egui temp memory, one frame old), where panels without a tab in the
+/// tabbed group go back to when docked.
+pub(crate) fn icons_rect_id() -> Id {
+    Id::new("floating-icons-rect")
+}
+
 pub(crate) fn tools_zone_id() -> Id {
     Id::new("floating-tools-zone")
 }
@@ -216,7 +222,13 @@ fn drop_target(app: &VectorcraftApp, ctx: &egui::Context, what: Moving, at: Pos2
             return Some((Drop::Stack(first), head));
         }
     }
-    zone(dock_rect_id()).filter(|_| app.ui.dock && app.ui.screen_mode < 3).map(|r| (Drop::Dock, r))
+    let dock = zone(dock_rect_id()).filter(|_| app.ui.dock && app.ui.screen_mode < 3)?;
+    // Panels that aren't tabs of the tabbed group (or all of them, with it collapsed) go back to
+    // their icons: light up the icon column, where they land, rather than the tabs.
+    let tabbed = !app.ui.dock_collapsed
+        && own.and_then(|i| app.ui.floating_panels.get(i)).is_some_and(|g| ids_of(g).into_iter().any(|p| DockTab::from_id(p).is_some()));
+    let lit = if tabbed { dock } else { ctx.data(|d| d.get_temp::<Rect>(icons_rect_id())).unwrap_or(dock) };
+    Some((Drop::Dock, lit))
 }
 
 /// Light up a drop zone: over the docked panels, under the floating ones (the moved one too).
@@ -262,7 +274,13 @@ pub fn track(app: &mut VectorcraftApp, ctx: &egui::Context) {
     let ids = ids_of(g);
     let active = ids.get(g.active).copied().unwrap_or(id);
     match drop {
-        Drop::Dock => dock(&mut app.ui, &ids, active),
+        Drop::Dock => {
+            dock(&mut app.ui, &ids, active);
+            // One that went back to its icon pops out of it, so it doesn't seem to vanish.
+            if app.ui.dock_collapsed || DockTab::from_id(active).is_none() {
+                app.ui.open_panel = Some(active.to_string());
+            }
+        }
         Drop::Stack(onto) => stack(&mut app.ui, &ids, active, onto),
     }
 }
@@ -607,7 +625,7 @@ mod tests {
         h.hold(h.title("layers"), over);
         assert!(h.group("layers").contains(over), "the group follows the pointer");
         let target = drop_target(&h.app, &h.ctx, Moving::Panel("layers"), over);
-        assert_eq!(target.map(|t| t.0), Some(Drop::Dock), "the dock is the drop zone");
+        assert_eq!(target, Some((Drop::Dock, dock)), "the dock is the drop zone, lit whole: its tab comes back");
         h.release(over);
         assert!(h.groups().is_empty(), "docked: {:?}", h.groups());
         assert_eq!((h.dock_tabs().len(), h.app.ui.dock_tab), (3, DockTab::Layers), "back in the dock, shown");
@@ -666,6 +684,24 @@ mod tests {
         assert_eq!(h.icons().len(), count - 1, "Color is back in the column");
     }
 
+    #[test]
+    fn a_panel_dropped_on_the_dock_lights_up_and_pops_out_of_its_icon() {
+        let mut h = Harness::new();
+        let count = h.icons().len();
+        h.drag(h.icons()[0].center(), pos2(500.0, 400.0));
+        assert_eq!(h.groups(), [["color"]]);
+        // Over the tabbed group: Color has no tab there, so the icon column lights up, not the tabs.
+        let (dock, column) = (h.temp_rect(dock_rect_id()), h.temp_rect(icons_rect_id()));
+        let over = h.dock_tabs()[1].center();
+        h.hold(h.title("color"), over);
+        assert_eq!(drop_target(&h.app, &h.ctx, Moving::Panel("color"), over), Some((Drop::Dock, column)));
+        assert!(column.width() < dock.width(), "the column, not the whole dock: {column:?} {dock:?}");
+        // Dropped, it's back in the column and pops out of its icon rather than vanishing.
+        h.release(over);
+        assert!(h.groups().is_empty());
+        assert_eq!((h.icons().len(), h.app.ui.open_panel.as_deref()), (count, Some("color")));
+    }
+
     #[test]
     fn a_popped_out_panel_dragged_by_its_title_floats() {
         let mut h = Harness::new();