Enhancement
In the context of the layout configuration #24085 it was noticed that no new tab can be created in the main window if an external window is open, because two different ContentHandlers are registered for the same ID.
If you want to open an external window, a new WindowComponent is instantiated using the WindowManager. The parent of the WindowComponent is set to the WindowManager's holder and thus added to the component tree.
Instead the new WindowComponent should be registered to the MainLayout of the Opener component.
Test
No test.