The bug described in this post is here again (C::B svn 12448, wxWidgets 3.1.5, Windows 10, but also with 12446 killerbot nightly). I obtain in any case something which looks like the Frame1 image in the first post, but just a small gray square
Changing the size and/or the font in Settings > Debugger > Choose button has no effect.
I have just added a Fit(); at line 1224 in watchesdlg.cpp and it works again.
At lines 1222 and 1223, the 2 m_sizer->Fit ... look strange. If I eliminate one of them or both, it continues to work. Are they necessary ? May be for other OS or wxWidgets version ... I don't know.
I have created a ticket #1100
here