I found a bug in the calculation of the minimum width of a dockable when using the Eclipse theme.
You can reproduce it in the “Themes” example from the framework.
In the eclipse theme you cannot move the main divider to the right. I think the minimum width of the
dockable is determined by the size of Yellow dockable tab.
You can however move it to the left so it does not seem to happen when the dockables are stacked.
In the other themes you can move the main divider to a very small minimum size independent of the dockable title - I think this is the desired behavior.