1.1.0p6e
Both dockables are initially shown side by side. When stacking them, the title bar is displayed wrongly.
Which would be the exact same error as this one. I’ll fix this in the evening, it can’t be a complex bug.
[Edit: And by now I know exactly what is going wrong. Some methods are called in an inconvenient order (the setting for showing the tab is made before the conditions are updated) There is no workaround, but it’s just some decoration and won’t affect the rest of the framework in any way. Any DockTheme that is not the EclipseTheme is not affected.]
Sorry for double-posting. I only checked the bugs forum before creating my post 
The fixed version is online. I did not fix the ConcurrentModificationException from the other thread as you said you already found the problem.