And the next one (the last one) happens much later. From further
testing, it seems to correlate to me typing 'C-x C-c', rather than to
when the frame finally refreshes after the delay (for example, when I
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
decide to stop waiting and hover with the mouse over the frame, or
^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
over the title bar buttons).
^^^^^^^^^^^^^^^^^^^^^^^^^^
Hovering over the title bar buttons generates damage on the window and
causes Mutter to ``wake up'', prior to anything further buffer flipping
by Emacs. Mutter cannot magically pull undisplayed frame contents out
of Emacs.