bug#51874: 27.2; [PATCH] `use-region-p' reports non-nil during simple `mouse-1' click

[Drew Adams]

The bug is detailed in emacs-devel thread "Fix for `use-region-p'
problem":

https://lists.gnu.org/archive/html/emacs-devel/2021-11/msg00634.html

In any case, I include a description here.

`use-region-p' is designed to let code know that either (1) the active
region is nonempty or (2) the user has OK'd the use of an empty region
(non-nil option `use-empty-active-region').  It should return non-nil in
those cases only.  The problem is in #2: an empty active region that's
declared to be OK to use.

A user (and code) expects `use-region-p' to return non-nil to show that
the region is usable for the _next_ user action (in particular).  In
simple user terms, in this context (being able to act on the active
region) a click of `mouse-1' can be considered a single user action.
There's no intention, and little ability, for a user to act _between_
the down & up events of a click (as opposed to a drag).

If `use-empty-active-region' is non-nil then the active region is usable
even if empty.  The bug is that `use-region-p' returns non-nil as soon
as you use `down-mouse-1'.  If the up-button event, `mouse-1' happens at
the same location then the region is deactivated.  But in the interim, a
call to `use-region-p' returns non-nil.

Especially if such a call is reflected by something user-visible, the
effect for users can be annoyance or confusion, or both.  For example, I
have code that shows, in the mode-line, the state of the region.  When
active, the region size is shown there, highlighted.  Clicking `mouse-1'
produces a flash, momentarily showing the empty region as active.  It
should not - a mouse click should not make the empty region "usable"
during the click.  Doing that is a mistake.

This recipe (from tomas@tuxteam.de) shows the problem.  Click `mouse-1'
to see that `use-region-p' reports non-nil during clicks.

(setq use-empty-active-region  t)

(setq my-timer
      (run-with-timer 0 0.05
                      (lambda()
                        (message "%s  REG=%s"
                                 (current-time-string)
                                 (use-region-p)))))

The fix I found is to have `use-region-p' not return non-nil for the
empty region if the last event is `down-mouse-1' or it satisfies
`mouse-movement-p'.

This is the fixed definition of `use-region-p' I came up with.

(defun use-region-p ()
  "Return non-nil if it is appropriate to act on the region.
This is used by commands that act specially on the region when
Transient Mark mode is enabled.

The return value is non-nil if the region is active and either of the
following conditions is true; otherwise, the return value is nil.
 * The region is nonempty.
 * Option `use-empty-active-region' is non-nil, and the last input did
   not use `mouse-1' to set or adjust the region.

For some commands, it might be appropriate to ignore the value of
`use-empty-active-region'; in that case, use `region-active-p'."
  (ignore-errors
    (and (region-active-p)
         (or (> (region-end) (region-beginning))
             (and use-empty-active-region
                  (not (eq 'down-mouse-1
                           (car-safe last-input-event)))
                  (not (mouse-movement-p
                         last-input-event)))))))

I'm no expert on events, mouse or otherwise, so if someone sees a better
implementation, great.  (I'm not sure the `ignore-errors' is needed, for
example.)

In GNU Emacs 27.2 (build 1, x86_64-w64-mingw32)
 of 2021-03-26 built on CIRROCUMULUS
Repository revision: deef5efafb70f4b171265b896505b92b6eef24e6
Repository branch: HEAD
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