retitle 51590 follow-mode is broken with header-line and tab-line
quit

>> The most low-level function of follow-mode is follow-calc-win-end.
>> When global-tab-line-mode is enabled, follow-calc-win-end
>> returns the same values as when global-tab-line-mode is disabled.
>
> Can you step through that function and see which part is misbehaving
> there?
>
>> I don't understand what more low-level function doesn't take
>> into account the height of the tab-line.  Maybe the problem is
>> in pos-visible-in-window-p?  Or maybe different values returned
>> by window-inside-pixel-edges and window-end?
>
> I don't know the answer, but I see that header-line-format is
> mentioned in one place in follow.el, and that is not a good sign...

Indeed, I tested with the header-line, and it has the same problem,
so retitled this bug report.

After trial and error, I arrived to the following patch that completely
fixes these problems.  However, I can't explain how it works.