Subject: bug#58176: 29.0.50; mouse-set-point region flicker with highlight-nonselected-windows=t and same buffer in two windows

From: <[email-masked]>
Date: Thu, 29 Sep 2022 23:09:37 +0200

Eli Zaretskii <[email-masked]> writes:

>> Date: Thu, 29 Sep 2022 19:38:28 +0200
>> From: miha--- via "Bug reports for GNU Emacs,
>>  the Swiss army knife of text editors" <[email-masked]>
>> 
>> 1. (setq highlight-nonselected-windows t)
>> 2. 'C-x 2'
>> 3. Click with mouse-1 somewhere to move point
>> 
>> Notice that for the duration mouse-1 being held down for the click,
>> region highlighting appears in the non-selected window. Usually the
>> click is short so the region only appears for a short time, which can be
>> perceived as a somewhat annoying flicker.
>
> It's a direct consequence of the fact that point is different in each
> window.  I don't see any bug here, FWIW.  If you don't like this side
> effect of highlight-nonselected-windows, then don't set it non-nil.

Well, maybe we could try a bit harder to reduce the flicker for ordinary
non-dragging mouse-1 clicks. One idea could be to prevent down-mouse-1
from activating mark until the region becomes non-zero length, so
something like this patch.

diff --git a/lisp/mouse.el b/lisp/mouse.el
index e38a4f8a71..a0f48de923 100644
--- a/lisp/mouse.el
+++ b/lisp/mouse.el
@@ -1729,7 +1729,7 @@ mouse-drag-track
                           '(only)
                         (cons 'only transient-mark-mode)))
           (let ((range (mouse-start-end start-point start-point click-count)))
-            (push-mark (nth 0 range) t t)
+            (push-mark (nth 0 range) t (not (eql (nth 0 range) (nth 1 range))))
             (goto-char (nth 1 range)))
 
           (setf (terminal-parameter nil 'mouse-drag-start) start-event)
@@ -1794,7 +1794,10 @@ mouse--drag-set-mark-and-point
 	   (goto-char beg))
 	  (t
 	   (set-mark beg)
-	   (goto-char end)))))
+	   (goto-char end)))
+    (unless mark-active
+      (unless (eql beg end)
+        (activate-mark 'no-tmm)))))
 
 ;; Commands to handle xterm-style multiple clicks.
 (defun mouse-skip-word (dir)