> IOW, there are several ideas of what the screen looks like:
> 1. the glyph matrix etc.
> 2. what Xlib has been told the screen should look like
> 3. what the screen actually looks like.
>
> update_frame synchronizes (2) with (1). x_sync synchronizes (3) with (2).
Yes, I know. But how does that let you know what update_frame
actually did?
It could do very little or nothing at all, and you need
to put a breakpoint at the right place (like the call to the
draw_glyphs method of the display interface) to see which parts are
being actually redrawn.