Problem: saving and restoring all frames to split-move is overkill now
that WinNewPre is not fired when split-moving.
Solution: defer the flattening of frames until win_split_ins begins
reorganising them, and attempt to restore the layout by
undoing our changes. (Sean Dewar)
704966c254
Adjust winframe_restore to account for Nvim's horizontal separators when the
global statusline is in use. Add a test.