redisplay issues

[Julian Bradfield]

Switching from 21.4.20 to 21.5.28, I notice numerous small breakages
in my VM session (running the same VM code (slightly patched 7.19) in
both cases).
Some of them I have some chance of hunting down (point being
mispositioned in the summary buffer, for example), but some not.

In particularly, redisplay seems very flickery. Frames with multiple
windows flicker every time certain changes are made to the buffers
(I can't be precise yet about what causes obvious flickers), and in
some operations the frame goes blank for a noticeable period while
XEmacs is doing some computation.
In general, it looks as if redisplay is happening when it ought to be
held off to wait for some change to complete, and then optimized.

Is this a recognized issue with 21.5? I've found no obvious reference
to it, though I haven't searched extensively.