Dock panel's currentTitle is not reset to undefined when the last widget is moved elsewhere

[lucas-koehler]

Bug Description:

After the last tab of a TheiaDockPanel in the shell (e.g. main or bottom) was moved to another area (e.g. main to bottom), the currentTitle property is not set to undefined but still points to the moved tab. It seems that the panel's markAsCurrent method is never called to reset to undefined.
This does not happen if the last widget is closed because the currentTitle is properly reset then.

The bug can be observed via the Close Tab in Main Area command. It is still active after the last widget was moved from the main to another area. Executing the command closes the moved widget despite it not being in the main area.

Steps to Reproduce:

1. Start with empty main area
2. Open editor
3. Move editor to bottom area
4. Execute command Close Tab in Main Area
5. The editor will be closed in the bottom area

Additional Information

close-main-tab.mp4

• Operating System: Ubuntu 20.04
• Theia Version: 1.23.0, commit 4411365