So after spending way too much time on this, and getting nowhere, I give up.  I'm attaching a patch that no dev is going to like because it behaves differently on Windows than it does on non-Windows to work around a know issue in GTK+ that is not documented to affect Windows differently than X windows.  Either someone else needs to find the root of the problem deep inside GTK+ (ie. why it reparenting only breaks on Windows), we figure out what's happening in Scintilla, we use the slightly hacky approach of behaving differently on Windows, or we just leave the whole plugin stay disable and let the Windows users not have it, even though we *can* make it working.  Since it's just a plugin, completely separate from the core code, which is already kinda hacky with what it's doing, my opinion is just to use the patch.

It's not my call, but the patch is here if you want it.