> What do you want to install on emacs-29, and what problem(s) is that
> supposed to fix?

This fixes a "wrong type error" when comparing
image-dired--number-of-thumbnails to zero.  Here is a recipe to make it
happen:

    - Do not have an existing *image-dired* buffer
    - Hit 'C-t a' (image-dired-display-thumbs-append) on an image file
      from Dired

And you're right that the rest of the patch should not go into emacs-29.
-- 
Manuel Giraud