bug#9609: Excessive transient region highlighting with highlight-nonselected-windows

[Eli Zaretskii]

> Date: Wed, 02 Jun 2021 15:43:25 +0300
> From: Eli Zaretskii <eliz@gnu.org>
> Cc: rgm@gnu.org, 9609@debbugs.gnu.org
> 
> > > emacs -Q --eval '(progn (setq highlight-nonselected-windows t) 
> > > (transient-mark-mode -1))' -f make-frame
> > >
> > > Two frames open showing the *scratch* buffer, once frame has focus.
> > >
> > > With the mouse, click on the "n" of "notes" in the focused frame. Do
> > > not release the mouse. In the other frame, a region starting or ending
> > > at "n" is now selected.
> > 
> > I can confirm that this is still present in Emacs 28.
> > 
> > Pretty weird behaviour.
> 
> Probably we decided that the mouse was dragged because the coordinates
> in the other window are different.  Or something like that.

Btw, you don't need another frame to demonstrate the issue.  You could
do this instead:

  emacs -Q --eval '(progn (setq highlight-nonselected-windows t) 
(transient-mark-mode -1))'
  C-x 2
  Click and hold the mouse button on some character