These are bugs, which I'm not sure how to deal with, and I've put them to the side.
Bugs which I expect to squash quickly are added to the top of todo.txt
Ctrl+Z While the Undo/Redo menus are open.
The Menu doesn't close, and will be buggy if a menu item is pressed.
Make window full-screen (on my high DPI laptop) and there's a lot of lag :-(
Need to add debugging timings to check where the bottleneck is. Probably ImageTransfer.
I thought ImageTransfer was quick enough (despite transferring the whole texture from GPU to CPU every frame)
Also check it textures etc. are being reused/freed appropriately.