Thank you for looking into this. I lost a couple of diagrams before I understood that this is indeed a bug. I made myself a small workaround for this moment by editing the keyboard shortcuts in such a way that Command-Q can not be choosen anymore from within Yed.
Within mac osx you can set all kinds of keyboard shortcuts at system level, and mac osx can have shortcuts assigned to particular programs only.
Currently I made the Command-Q command behave like Command P (which brings up a dialog box for printing). This workaround makes that I have no negative effect (for the moment) when accidently pressing Command-Q.
The 'cancel' button from the print dialog is working as expected.
If I need to quit the program, after disabeling the Command-Q the only possible way to Quit is to actually point towards the quit menu.
This helps me (and possible others) to avoid unexpected quit-behaviour.
Hope this workaround will help others as well that run into the same problem.
(Place to set the shortcuts: System menu > Keyboard > Keyboard combinations > make a fake menu item (or point to print menu) and set Command-Q to be the key combination that is used..
Make the key-combination available to Yed only, otherwise the Command-Q is unavailable in all programs :-)