I admin several wikis on Miraheze and Timeless is my skin of choice in majority of cases. The farm just updated MediaWiki to 1.42 and I noticed source editor started behaving differently in Timeless - it's trying to fit the entire editor, with bottom block (with summary/save/show/cancel) into window size (either actually windowed or full screen on my modest laptop), resulting in pretty small actual editor area, while it wasn't an issue before. Disabling all custom CSS/JS doesn't do anything, the issue remains. It doesn't replicate here tho, but mw.org is on 1.43. Was it a legit problem that got fixed or it's something funky on farm's side?
UPD: With another user came to conclusion that there's something with CodeMirror extension; turning off syntax highlight brings normal sizing back, however some other Timeless wikis on Miraheze with syntax highlight don't have such problem.