bug#37667: 27.0.50; Tab Bar display problems with more than 5 tabs

[Juri Linkov]

>> The main problem in implementing auto-scrolling is how to detect the
>> situation when the current tab is not visible.  Maybe by implementing
>> a function like pos-visible-in-window-p, but named tab-visible-in-tab-line?
>
> That'd be fine with me.  Let me know if you need help in making that
> happen, either by talking or by coding.

I'm still trying to understand why truncation flags are not set in the tab-line.
Test case:

0. emacs -Q
1. M-x global-tab-line-mode RET
2. C-x b 01234567890123456789012345678901234567890123456789 RET
   more buffers with long names might be needed
   to cause the tab-line truncation

3. M-x dump-glyph-row RET

shows no truncation:

Row     Start       End Used oE><\CTZFesm     X    Y    W    H    V    A    P
==============================================================================
  0         0         0  111 110000000000     0    0  677   16    0   12    9

i.e. the columns '>' ('row->truncated_on_right_p')
and '<' ('row->truncated_on_right_p') both have values '0'.

Is this a bug?  Or maybe there is another way to detect tab-line truncation
in the glyph matrix?