./myapp -disableaccurateframe can be used as workaround.
Thanks hansotten, useful info.
Fortunately, the more correct --disableaccurateframe also works. The gnu standard says single char options have one dash, multichar options should have two. In our case Application.CheckOptions() knows about only ShortOptions and LongOptions, no hybrids ! So, it will report the one dash version as an error, app should exit.
And interestingly, I can reproduce this problem (of the little window) only with gtk2, Qt5 works fine.
Davo