This is a pretty obscure bug, and I am unsure of what exact conditions triggers this but it seems to have to do with setting a toolwindow in a tabbedMDIstate to a dockable state when you have split documents - the steps below are the only reliable way I have found to duplicate the bug in the demo project so far.
1) Open the dock demo project
2) Close all windows (definitely occurs without this step, but not in reproducible way for me...in the demo at least)
3) Create 3 Tool windows attached
4) Center dock all 3 at once into tabbedMDIDocuments
5) Move toolwindow1 into its own horizontal tab group
6) Make toolwindow3 dockable
7) Notice that Toolwindow1 is now the active tabbedMDIdocument (this behaviour is correct, but bug occurs whenever this happens). Shift focus to toolwindow2
8) You can no longer bring toolwindow1 to focus (title won't change to bold, and focus event never fires)
1) Open the dock demo project
2) Close all windows (definitely occurs without this step, but not in reproducible way for me...in the demo at least)
3) Create 3 Tool windows attached
4) Center dock all 3 at once into tabbedMDIDocuments
5) Move toolwindow1 into its own horizontal tab group
6) Make toolwindow3 dockable
7) Notice that Toolwindow1 is now the active tabbedMDIdocument (this behaviour is correct, but bug occurs whenever this happens). Shift focus to toolwindow2
8) You can no longer bring toolwindow1 to focus (title won't change to bold, and focus event never fires)
