Your hunch was dead on, Kai. Changing the theme fixes the bug. My theme was not custom, though, it was the default lubuntu theme. The bug repros with themes "lubuntu" and "lubuntu-dark-panel" but not with "industrial" or "redmond" or any others. For "lubuntu" and "lubuntu-default", there are a few logs on stdin which I've uploaded to
https://paste.ubuntu.com/24520462/ and
https://paste.ubuntu.com/24520461/ respectively.
1/ Do you think it is worth filing a bug against lubuntu 17.04?
2/ If you know of any other app that uses gtksourceview (I have 3.0-1 installed), I can test that app to see if it repros with all gtksourceview controls or if it is specific to meld's use of gtksourceview.