bug#73231: 30.0.91; image-dired cannot be operated until all thumbnails are created (MS-Windows)

[Eli Zaretskii <eliz@gnu.org>]

> From: AKIYAMA Kouhei <misohena@gmail.com>
> Date: Fri, 13 Sep 2024 22:07:48 +0900
> 
> When using image-dired without ImageMagick or GraphicsMagick
> available, operations could not be performed until all thumbnails had
> been created.

Thank you for your report and the research that went into it.

When you speed up the display of the buffer, don't you see in
*Messages* error messages about missing files that cannot be
displayed?  The default values of delays and timers were chosen so as
to minimize these error messages for some reasonable amounts of images
in a directory (your 274 images is way larger than what I had in
mind).

image-dired--probe-thumbnail-cmd is called frequently because the user
could install ImageMagick at some point, and I didn't feel like adding
complicated logic to allow detection of that that is cheaper.  But if
you have practical suggestions for how to make these tests more seldom
without disabling them completely, that could be a good improvement.

In general, the non-ImageMagick implementation was intended to be a
fallback, for those who cannot or will not install ImageMagick.  It is
deliberately synchronous, but I think it performs well enough to be
usable.

Granted, improvements to the code are welcome, but if they make the
redisplay error messages appear in more cases, I think such a change
would be for the worse, because users can rightfully think Emacs has a
bug.