bug#29478: [Patch] bug#29478: 26.0.90; `C-h k' followed by mouse clicks no longer shows down event

[Alan Mackenzie <acm@muc.de>]

Hello, Eli.

On Sat, Dec 23, 2017 at 11:42:10 +0200, Eli Zaretskii wrote:
> > Date: Fri, 22 Dec 2017 22:05:49 +0000
> > Cc: drew.adams@oracle.com, 29478@debbugs.gnu.org,
> >   npostavs@users.sourceforge.net
> > From: Alan Mackenzie <acm@muc.de>

> > (i) emacs-26 -Q
> > (ii) M-: (read-key-sequence "prompt: ") <CR>
> > (iii) <shift>-mouse-1

> > What happens now is the key sequence is displayed, a S-down-mouse-1
> > event, but immediately overwritten in the message area by 

> >     <S-mouse-1> is undefined

> > .  Why?  I merely asked for the key sequence.  I didn't ask for it to be
> > looked up in any key maps.  What is doing the translation from a
> > shift-down-mouse event to a S-click event?  What is looking up this
> > down-mouse event in a key map?

> read-key-sequence itself examines the possible remapping of the
> sequence.  And it seems reasonable, since how can it otherwise know
> when the sequence is complete?

> What is perhaps unexpected here (or might be a bug) is that for some
> reason the sequence is left in unread-command-events (or somewhere
> similar, perhaps in read_key_sequence_remapped?), because if I set a
> breakpoint in Fding, the backtrace from the "is undefined" message
> clearly shows it was the command loop that invoked 'undefined':

[ .... ]

Of course!  read-key-sequence consumes all the events up to the
S-down-mouse-1, leaving the following S-mouse-1 in the event buffer.
This is read at the next iteration of the command loop.

> CC'ing Stefan in the hope that he could have some insights.

OK, I have a provisional fix.  The problem was that the double-click-time
loop in help-read-key-sequence was discarding all events but the last
one.  I have amended the loop so that _all_ these events are stored in a
list, and we then discard the most recent events till we find one with a
binding.

This patch seems to work with GPM in a Linux tty, and also in X-Windows
(I think).  (It is based on the emacs-26 branch, of course.)



diff --git a/lisp/help.el b/lisp/help.el
index 212e3679da..dd1676adb0 100644
--- a/lisp/help.el
+++ b/lisp/help.el
@@ -717,7 +717,7 @@ help-read-key-sequence
         (cursor-in-echo-area t)
         saved-yank-menu)
     (unwind-protect
-        (let (key down-ev)
+        (let (key keys down-ev discarded-up)
           ;; If yank-menu is empty, populate it temporarily, so that
           ;; "Select and Paste" menu can generate a complete event.
           (when (null (cdr yank-menu))
@@ -731,6 +731,7 @@ help-read-key-sequence
                  (or
                   (and no-mouse-movement
                        (string-match "mouse-movement" keyname))
+                  (progn (push key keys) nil)
                   (and (string-match "\\(mouse\\|down\\|click\\|drag\\)"
                                      keyname)
                        (progn
@@ -739,13 +740,31 @@ help-read-key-sequence
                          (sleep-for 0.01)
                          (while (read-event nil nil 0.01))
                          (not (sit-for (/ double-click-time 1000.0) t))))))))
+          ;; When we have a sequence of mouse events, discard the most
+          ;; recent ones till we find one with a binding.
+          (let ((keys-1 keys))
+            (while (and keys-1
+                        (not (key-binding (car keys-1))))
+              ;; If we discard the last event, and this was a mouse
+              ;; up, remember this.
+              (if (and (eq keys-1 keys)
+                       (vectorp (car keys-1))
+                       (let* ((last-idx (1- (length (car keys-1))))
+                              (last (aref (car keys-1) last-idx)))
+                         (and (eventp last)
+                              (memq 'click (event-modifiers last)))))
+                  (setq discarded-up t))
+              (setq keys-1 (cdr keys-1)))
+            (if keys-1
+                (setq key (car keys-1))))
           (list
            key
            ;; If KEY is a down-event, read and include the
            ;; corresponding up-event.  Note that there are also
            ;; down-events on scroll bars and mode lines: the actual
            ;; event then is in the second element of the vector.
-           (and (vectorp key)
+           (and (not discarded-up) ; Don't attempt to ignore the up-event twice.
+                (vectorp key)
                 (let ((last-idx (1- (length key))))
                   (and (eventp (aref key last-idx))
                        (memq 'down (event-modifiers (aref key last-idx)))))


-- 
Alan Mackenzie (Nuremberg, Germany).