Eli Zaretskii wrote:
> You'd need to explain how Emacs succeeds in that, when it uses Xlib
> and higher-level APIs, which AFAIK are unaware of any accelerations.
> Emacs itself is certainly unaware of that, and does the same things
> regardless.

The Emacs algorithms for redrawing the frame's content could be based on
assumptions about the workings of graphical displays that fail to be
true in some corner cases.

> Moreover, the fact that running xrefresh, which is not an Emacs
> command, fixes the display is yet another argument against this
> hypothesis.  xrefresh doesn't communicate with Emacs, so the only way
> it could fix the display is if the data supplied by Emacs was correct.

Of course 'xrefresh' command doesn't communicate directly with Emacs.
It works by mapping a window, with no background, on top of the Emacs'
frames, and then unmapping it, causing the X server to send a refresh
event to Emacs, that handles it and repaints its frames.  So Emacs *do
know* how to get its frames right, when it wants to.

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