First of all, glad it's working out now.
Not a specific plan on updating L&F, but will create a ticket.
This might be another solution to your issue if you prefer working on Mac OS X L&F.
Go to [Tools] - [System Properties] - [Diagram Editor] and check [Hide title bar of the Diagram Editor] option.
Restart Astah and switch the L&F back to Mac OS one.
FYI, this is the diagram editor without title bar:
this is the one with title bar (it is on by default)
Let me know if this works!