Add a new layer. Press 'U' to undo. Press 'U' to undo the undo. System freezes.
[679] Undoing adding layers
Confirmed in 679.
The undo framework was never really designed to handle undoing changing the layer properties of a screen - for now I've changed the layer dialog so that it is no longer undoable in the next build.
If we ever rewrite ZQ we will give it a more comprehensive Undo system.