Redisplay problems; request for help

[Jim Blandy]

I've been seeing a problem with redisplay that I can't quite
reproduce; if someone else has a reliable method, I can try to debug
it.

When a window's start changes, either due to something like an
i-search moving to a match some distance down the buffer and redisplay
recentering the display, or due to IRC conversation rolling off the
bottom of the window, some areas of the screen that should be cleared
occasionally have text from the prior rendering left in them.  C-l
redraws things correctly, but as the incorrect characters are drawn
from (another part of) the same buffer being displayed, the corruption
is not always easy to detect.

Unfortunately, I haven't found a way to make this happen reliably.  If
you have seen this and know how to make it go, please let me know.