SourceTree crashed, and this showed up in a terminal window on restart:
Robs-Mac-Pro:~ robweaver$ /Applications/Sourcetree.app/Contents/MacOS/Sourcetree ; exit; 2021-07-23 08:35:33.560 Sourcetree[19877:2469132] It's not legal to call -layoutSubtreeIfNeeded on a view which is already being laid out. If you are implementing the view's -layout method, you can call -[super layout] instead. Break on void _NSDetectedLayoutRecursion(void) to debug. This will be logged only once. This may break in the future.