Re: Mysterious redisplay problem and a trap in xdisp.c

[Richard Stallman]

    > While typing in some text in a buffer, emacs suddenly decides to
    > insert a new line instead of the character I typed.  But if I do C-l,
    > there is only one line, and the character I typed magically reappears.

Actually it did not "insert a newline".  This evidence proves that.
The bug is in redisplay.

    If I comment out the call to make_gap_smaller in make_gap, the problem
    goes away!

This may be of some help, but the reliable way to track this down is
to step thru redisplay when the problem happens.  Anything else is
just guesswork.  There is a chance this "clue" will lead someone to
think of a possible cause, but if that doesn't happen, the reliable way
is likely to be the fastest way.

Don't waste your time staring at make_gap_smaller.  You know what
it does.  That's not where the problem is.

Here's what it makes me think of: is there anything in redisplay that
points to a buffer character by its address rather than by its
character number?  Anything that might live across the computation of
the mode line?

Is the problem reliably reproducible now?  If not, maybe you can make
that happen by having some function called from mode-line-format
execute (garbage-collect).