bug#45628: 28.0.50; Scrolling on echo area with pixel-scroll-mode enabled locks up Emacs

[Eli Zaretskii]

> From: Amin Bandali <bandali@gnu.org>
> Date: Sun, 03 Jan 2021 11:14:35 -0500
> 
> As the Subject says, scrolling using the mouse wheel on the echo area
> causes Emacs to freeze/lock up (cursor is changed to spinning circle),
> when pixel-scroll-mode is enabled.
> 
> I can reproduce in emacs -Q after evaluating the following in *scratch*:
> 
> (require 'pixel-scroll)
> (pixel-scroll-mode 1)

Thanks.

Tak, can you please look into this?  The code infloops inside
pixel-scroll-down, here:

        (while (pixel-point-at-bottom-p amt) ; prevent too late (multi tries)
          (vertical-motion -1))              ; move point upward

because the mini-window is normally just one screen line high, so its
top and bottom coincide.

Shouldn't we exit the loop when vertical-motion returns zero, which
means it didn't move?

A similar loop exists in pixel-scroll-up, AFAICS.