bug#2895: 23.0.92; Tooltip persists with tooltip-mode off (new syndrome)

[Bob Rogers]

   This can be reproduced in trunk, built as of a few hours ago, and
requires circumstances different from bug#2680, though the buggy symptom
is similar:  A tooltip is redisplayed in the message area after moving
the mouse to a spot where it no longer applies.

   To reproduce:

   1.  "emacs -Q"

   2.  "M-x tooltip-mode RET".  This is on by default, so it should
display "Tooltip mode disabled" in the message area.

   3.  Invoke "C-e" (or whatever) in order to clear the message area;
the bug doesn't manifest reliably if something is already displayed
there.

   4.  Move the mouse into the "-----" part of the right edge of the
"*scratch*" buffer mode line.  The message area will show the tooltip
correctly, combined into a single line and (if the frame is narrow
enough) truncated at the right.

   5.  Slide the mouse to the left until it enters the "Lisp
Interaction" area, without taking the mouse out of the mode line.  The
message area will change to show a different tooltip for the highlighted
"Lisp Interaction" area.  It is important that the two mouse-sensitive
areas be adjacent, with no intervening "dead" area, so not all mode line
components are suitable for this.

   6.  Now move the mouse back into either the *scratch* buffer or the
message area.  Instead of reverting the message area to the pre-tooltip
message, the tooltip for "-----" will be redisplayed.

   I was able to fix this superficially, but the resulting code was
subject to timing problems, and it eventually lost any previous message,
so no patch this time.

   FWIW, one timing problem happened when mouse jitter appeared to make
the cursor move from mode line to message area and back three times in
an eyeblink; the result was a regression of bug#2680.  I managed to
reproduce this twice, after lots of trying.  It looks like it could be
due to a failure to execute tooltip-show-help-non-mode atomically, which
makes me think I don't understand enough about how mouse motion events
are processed to produce a solid fix.

                                        -- Bob Rogers
                                           http://www.rgrjr.com/