bug#46320: 27.1; bugs about scrolling buffer with images

[Eli Zaretskii]

> From: ynyaaa@gmail.com
> Date: Fri, 05 Feb 2021 14:45:34 +0900
> 
> (1)wrong display
> Evaluate the form below, only the line 20 is displayed.
>  Type C-p, the buffer scrolls down and the lines 2 to 17 are
> displayed and the cursor is displayed at the end of line 2.
> At this time, the real current point must be at the beginning of the
> line 19.
> Type C-p again, the line 18 is displayed at the top of the window.
> 
> (let* ((buf (generate-new-buffer "tmp"))
>        (width 80)
>        (height 350)
>        (svg (format "\
> <svg width=\"%d\" height=\"%d\" version=\"1.1\"
>  xmlns=\"http://www.w3.org/2000/svg\";>
> <rect x=\"0\" y=\"0\" width=\"%d\" height=\"%d\" fill=\"blue\"/>
> </svg>
> " width height width height))
>        (img (create-image svg 'svg t :scale 1)))
>   (switch-to-buffer buf)
>   (setq header-line-format "with header line")
>   (insert-image img "1")
>   (insert "\n2\n3\n4\n5\n6\n7\n8\n9\n")
>   (insert-image img "10")
>   (insert "\n11\n12\n13\n14\n15\n16\n17\n18\n19\n20\n")
>   (set-frame-height nil 40)
>   (delete-other-windows)
>   (forward-line -1)
>   (set-window-start nil (point))
>   )

This is already fixed in Emacs 28 (the master branch).

> (2)unable to scroll up

Please don't report more than one problem in each bug report, even if
they are about similar issues.  Please make a separate report about
each problem.  TIA.