Do not set the dockpanel as parent of the tabbar (#606)

* Add a focusTrackable attribute to Widget, to disable the focusTracker

* Add tests on trackable attribute

* Revert "Add tests on trackable attribute"

This reverts commit 9324ea6.

* Revert "Add a focusTrackable attribute to Widget, to disable the focusTracker"

This reverts commit a43945d.

* Attach the tabbar to the dock panel without setting its parent

* Add test on tabbar not parented to docklayout

* Add comment in test

packages/widgets/src/docklayout.ts

@@ -1128,9 +1128,8 @@ export class DockLayout extends Layout {
     // Enforce necessary tab bar behavior.
     tabBar.orientation = 'horizontal';
 
-    // Reparent and attach the tab bar to the parent if possible.
+    // Attach the tab bar to the parent if possible.
     if (this.parent) {
-      tabBar.parent = this.parent;
       this.attachWidget(tabBar);
     }
 

packages/widgets/tests/src/dockpanel.spec.ts

@@ -44,6 +44,16 @@ describe('@lumino/widgets', () => {
         let panel = new DockPanel();
         expect(panel.hasClass('lm-DockPanel')).to.equal(true);
       });
+
+      it('should not have tabbar as child', () => {
+        let panel = new DockPanel();
+        // Adding a widget in the dock panel adds the DOM of a TabBar, but the TabBar
+        // widget should not be a in the children list of the DockPanel widget.
+        panel.addWidget(new Widget());
+        for (const tabBar of panel.tabBars()) {
+          expect(panel.contains(tabBar)).to.be.false;
+        }
+      });
     });
 
     describe('#dispose()', () => {