bug#20157: 25.0.50; `describe-key-briefly' shows byte-code for <mouse-movement>

[Drew Adams <drew.adams@oracle.com>]

I cannot easily give a recipe to repro this.  But using
`describe-key-briefly' with non-nil INSERT arg and clicking mouse-1
in some contexts gives me this:

<down-mouse-1> at that spot runs the command mouse-drag-region
<mouse-movement> runs the command #[257 "..." [(down-mouse-1
(#<window 26 on throw-show-key.el> 5007 (577 . 198) 152173811 nil
5007 (82 . 16) nil (3 . 6) (0 . 12))) (5007)
#<window 26 on throw-show-key.el> 0 44 0 nil event-end posn-point
mouse-movement integer-or-marker-p mouse--drag-set-mark-and-point
mouse-position mouse-scroll-subr nil auto-hscroll-mode] 9 "

(fn EVENT)" "e"]

where the "..." after the #[257 is really byte-code.

So far, I have not seen byte code for other key descriptions.

I say in some contexts because if I just put this in a buffer and use
C-x C-e on it, it just inserts "<mouse-movement> runs the command
ignore":

  (describe-key-briefly [mouse-movement] t)

So far, I have not seen byte-code for other key descriptions.

I tried using the debugger on this, but was unable to repro the problem
then.  Apparently, sometimes clicking mouse-1 causes the normal text
"runs the command ignore" to be used.  Sometimes it shows the byte code.

The preceding command, when it does this, seems always to be
`mouse-drag-region', from a <down-mouse-1> event.