Why doesn't `scroll-margin` work? [Solved]

Question

• scroll-margin is 1 but Emacs still sometimes allows the cursor to stay at the 1st line of a window.

Follow these steps to reproduce it (or see the GIF below):

1. emacs -Q

2. In buffer *scratch*, C-x C-e:

   (progn
     (custom-set-variables
      '(custom-enabled-themes '(modus-vivendi))
      '(global-tab-line-mode t)
      '(scroll-margin 1)
      '(scroll-conservatively 101))
     (info-emacs-manual)
     (end-of-buffer)
     (goto-line 500))
   

3. With the cursor now at the 2nd line from the top of the window, press <up>-arrow-key several times and pay attention to resulting cursor position.

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This behavior is inconsistent with the documentation (and has caused me great trouble in practice):

C-h v scroll-margin:

Its value is 1
Original value was 0

Number of lines of margin at the top and bottom of a window.

The correct behavior is what next-line does;
You can try the <down>-arrow-key after step 3 described above:

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Q:

Why doesn't the setting of scroll-margin work here?
IOW, what causes previous-line to move the cursor beyond the margin (which is inconsistent with next-line's behavior when Emacs automatically scrolls up)?

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Information that may be helpful in troubleshooting:

• If either '(custom-enabled-themes '(modus-vivendi)) or '(global-tab-line-mode t) is removed, the bug will disappear.

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emacs-version: 28.2
system-configuration: x86_64-w64-mingw32

emacs-version: 30.0.50
system-configuration: x86_64-pc-linux-gnu

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Question is Solved

I reported this as bug#63201 and it should be fixed now on the emacs-29 branch.