bug#43379: [PATCH] Double-click events can occur without preceding single-click events

[Eli Zaretskii <eliz@gnu.org>]

> From: Daniel Koning <dk@danielkoning.com>
> Date: Sun, 13 Sep 2020 12:00:10 -0500
> 
> ++++
> +** Repeat events are now produced only when the modifier keys are the same.

What will this do if the modifier key is "simulated" using "C-x @"?

More generally, for a change this deep and wide, I'd like to
understand better what exactly are the problems being solved, and also
how can we be sure (by testing or otherwise) we are not getting
regressions in some use cases.  Could you please clarify that?

> +Before, when the user pressed the same mouse button repeatedly within
> +the bounds specified by 'double-click-fuzz' and 'double-click-time',
> +it always produced a 'double-' or 'triple-' event, even if the user
> +was holding down modifier keys on one click and not another.  This
> +meant that it was possible for Emacs to read a double-click event
> +without reading the same kind of single-click event first.  Emacs now
> +looks at modifier keys to determine if a mouse event is a repeat.

Beyond theoretical (un)cleanliness, what other practical problems did
you find with the current code and fix in these patches?

>  This variable is also the threshold for motion of the mouse to count
> -as a drag.
> +as a drag.  (But if the mouse moves from one screen position to
> +another while the button is held down, it always counts as a drag, no
> +matter the value of @code{double-click-fuzz}.)

Isn't this an incompatible change?

Thanks.