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

[Stefan Monnier]

>> There's one dog we cannot let lie: the one that fails the test in
>> completion-preview-tests to fail to compile -- this is what started
>> this bug report, and I would like to solve that problem.
>
> From a cursory reading of this bug report, I gather that m-w-*-event and
> m-w-*-alternate-event are at times defined to identical values, which
> causes defvar-keymap to signal?  In that situation the correct course of
> action is using [remap ...] to bind the mwheel commands to whatever
> completion-preview needs it set to, right?

I considered doing that, but it's not great:

- The same command is used for up and down, so we'd have to change the
  completion-preview code to have a new command that looks at the
  invoking event to decide whether to scroll up or down.
- The same command is also used for left/right, which we currently
  don't override in completion-preview, but which a `remap` would
  also affect.
- I'm not sure it's quite the semantics we want (I don't think we want
  these completion-preview bindings to depend on
  `mouse-wheel-scroll-amount`).

None of those problems are deal breaking, of course, but overall it's
not a much better option.  And I suspect that once we sort out the
m-w-*-event problems, fixing the completion-preview problems will be
even easier.


        Stefan