bug#35899: 26.1; move-beginning-of-line sometimes does not move to beginning of line

[Markus Triska]

Please download hello.png with:

    $ wget https://www.metalevel.at/ei/hello.png

and then, starting Emacs with "emacs -Q", evaluate the following form:

    (progn
      (goto-char (point-min))
      (save-excursion (insert "\n"))
      (goto-char (+ (point) (cadr (insert-image-file "hello.png"))))
      (move-beginning-of-line nil))

This yields: 592, and does not move point to the beginning of the line.

However, the documentation of move-beginning-of-line includes:

   (If there’s an image in the line, this disregards newlines
   which are part of the text that the image rests on.)

Hence, I expect this to move to position 1. For comparison, when I press
C-a, it does move to position 1, as expected.

Would you please consider adjusting the function so that it behaves as
documented? Alternatively, could you please provide this feature in a
different way?

Thank you and all the best,
Markus

In GNU Emacs 26.1 (build 3, x86_64-pc-linux-gnu, X toolkit, Xaw scroll bars)
 of 2019-04-09 built on mt-laptop
Windowing system distributor 'The X.Org Foundation', version 11.0.11906000