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

[miha--- via "Bug reports for GNU Emacs, the Swiss army knife of text editors" <bug-gnu-emacs@gnu.org>]

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Eli Zaretskii <eliz@gnu.org> writes:

>> (I'm assuming that by "show regions of zero length", you mean that the
>> "Copy text" toolbar icon is enabled.)
>
> No, we actually attempt to display zero-length regions in the text.

Maybe there's something obvious I'm missing, but in a graphical emacs
-q, I can't see a zero length active region in the text at all. That is,
after activating it with C-SPC or a non-dragging mouse-1 button press, I
can't see a single pixel around point changing to indicate the region. I
even tried it after considerably enlarging the font with 'C-x C-+ + + +
+ ...'.

(On the other hand, zero-length rectangular regions, created with C-x
SPC, are properly indicated with a thin vertical line, but we aren't
talking about rectangular regions here.)

>> I hope I'm not arguing semantics too much, but my patch doesn't actually
>> hide an (active) region of zero length, it prevents one from being
>> activated.
>
> Which is against the behavior we want, I think.
>
>> It only prevents it in the specific case of a non-dragging mouse-1
>> click, which makes quite a lot sense to me. The current behaviour of a
>> non-dragging mouse-1 click is that the zero-length region is only active
>> for the duration of the mouse button being held down. Once the button is
>> released, the zero-length region is deactivated, which makes it not very
>> useful, as far as I can see.
>
> It is useful in the much more frequent use case where the user then
> drags the mouse.

This use case is, of course, still supported by my patch.

I was talking about the very specific zero-length region, active for the
duration of a non-dragging mouse-1 button being held down. My patch
affects only this specific use-case

>> If you want to create a zero-length region that stays active after
>> releasing mouse-1, you have to drag the mouse a little first to make the
>> region length non-zero and then drag the mouse back to the starting
>> position to make the region length zero again. This use case is still
>> supported by my patch.
>> 
>> (Perhaps I missed some use-cases of a zero-length region active during a
>> non-dragging mouse-1 button hold. If you know any, please let me know
>> about them.)
>> 
>> This, together with the fact that it reduces the "flicker" from the
>> original message, makes me insist on the patch.
>
> I still insist that what you propose is the opposite of the behavior
> we want.

Another unrelated thing to consider is the current behaviour of
clicking/dragging C-M-mouse-1. It doesn't activate a rectangular region
until the user drags C-M-mouse-1. In fact, if the user only does a
non-dragging C-M-mouse-1 click, the rectangular region is never
activated, only the point moves. You can try experimenting with
C-M-mouse-1 and see, if this is really the opposite of the behaviour we
want.

In essence, my patch tries to make behaviour of mouse-1 region
activation more similar to C-M-mouse-1 rectangular region activation.

(Emacs build info follows, since the non-display of zero-length
non-rectangular regions in the text could be dependent on build
configuration.)

In GNU Emacs 29.0.50 (build 2, x86_64-pc-linux-gnu, GTK+ Version
 3.24.34, cairo version 1.17.6) of 2022-09-29 built on miha-pc
Repository revision: 0edd7770e0ce70cac59f239134962d10f48dec79
Repository branch: master
Windowing system distributor 'The X.Org Foundation', version 11.0.12101004
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