bug#68213: 30.0.50; completion-preview-tests failure in --without-x build

[Po Lu]

Stefan Monnier <monnier@iro.umontreal.ca> writes:

> Any chance you could try and help me figure out answers to those questions?
>
> At the very least we have a documentation problem because, as a user
> reading the docstring of `mouse-wheel-down-alternate-event`, I have no
> idea what this means or does and how it differs from
> `mouse-wheel-down-event`.
>
> `mouse-wheel-down-event` is a defcustom because X11 failed
> to define events for scroll wheel, and depending on your mouse, those
> were not always mapped to the same `mouse-N` events.  That doesn't seem
> to apply to `mouse-wheel-down-alternate-event`.

mouse-wheel-*-alternate-event were initially meant to reflect a second
type of event generated by the window system in response to scroll wheel
motion.  At the time, it was set on XInput2 builds and no more, where
wheel-* events are generated in addition to the customary mouse-*
events.

This arrangement was later modified before the release to set
mouse-wheel-*-alternate-event to mouse-* wherever it had previously been
nil, to provide for xterm-mouse-mode generating such events whatever the
default value of mouse-wheel-*-event.

As such, I suppose the difference is that between the mouse event
generally emitted by the window system, and events which might be
encountered under special circumstances.

> Do you happen to remember the bug numbers of bug titles or something to
> help me find info about those obscure problems?

I think searching the bug tracker for "xterm-mouse-mode" and "mwheel"
will help, and I also recall that the PGTK port was involved in some
manner.