Re: mode-line redisplay bug

[Edward O'Connor]

Several months ago, I wrote:

> Every so often for a while I've occasionally seen this odd redisplay
> bug, and I now think I know about it to report it. Anyway, here's what
> happens: I set `mode-line-format' to nil in my eshell buffers. I
> switch to an ERC buffer with the command `erc-track-switch-buffer'
> (which is just a cute wrapper around `switch-to-buffer'). My mode-line
> in the ERC buffer upon such a buffer switch will often be garbled,
> with only part of it appearing, as you can see in this screenshot:
>
> http://edward.oconnor.cx/tmp/emacs/bug1.png
>
> I thought that the bug might have something to do with my custom
> `mode-line' face, but as you can see in this other screenshot, it
> occurs with default Emacs faces as well:
>
> http://edward.oconnor.cx/tmp/emacs/bug2.png
>
> (For completeness sake, here's what a normal ERC buffer looks like:
> http://edward.oconnor.cx/tmp/emacs/normal.png)
>
> I imagine the redisplay code is trying to only redraw those parts of the
> mode-line that have changed, or something like that. Nevertheless, it
> does seem that the redisplay engine doesn't realize that, when switching
> from a buffer without a mode-line, *everything in the mode-line* needs
> to be redisplayed.

(For the record, I've seen this bug under X11, Mac OS X, and w32.)

Richard Stallman replied:

> Can you please send a precise self-contained test case for this bug?

And I've finally written a self-contained test case (attached):

redisplay-bug.el
Description: test case for redisplay bug