serious redisplay bug!

[Richard Stallman]

In an 80x40 X frame, I did M-x customize-group RET files RET, then C-x 4
b *scratch*.  With *scratch* selected, and with point at the beginning
of the customization buffer, I typed C-M-v.

The top two and a half lines of the customization buffer remained
on the screen.  They should have been replaced by text that was
scrolled up.

Later I tried simply doing C-u C-v in the customization buffer
with point at the top.  It displayed wrong too.

Would you please take a look at it?


This is with no init file, and with the sources as of around 0330 Dec 1 GMT.
The problem does NOT happen in the release branch.  Maybe someone can track
it down quickly by looking for recent redisplay changes.