[GREENFOOT-855] Greenfoot editor: Tab selection menu unusable when many files open in editor

Under Linux Mint Xfce:

Open many files in the Greenfoot editor (so many that their titles fill the window's width)
click the downwards-pointing triangle button that appears to the right (I presume it is supposed to offer a tab selection menu?)
the menu does open, but it is only a few pixels wide and seems to have a number of empty entries;
the menu does have the correct number of entries, and tab selection does work. It's just not rendered correctly.

I apologize for my stupidity, but I seem to be unable to attach a screenshot in Jira.

Issue metadata