bug#27895: 26.0.50; display-line-numbers and header-line-format alignment

[Eli Zaretskii <eliz@gnu.org>]

> From: Mark Oteiza <mvoteiza@udel.edu>
> Date: Mon, 31 Jul 2017 21:35:48 -0400
> 
> Enabling display-line-numbers should affect how header-line-format is
> shown.

I'm not sure I agree with this, not in this general form.  Modes use
header-line in different ways, and I don't think we can decide for all
of them up front how they should react to line numbers.

While I agree that tabulated-list modes and ruler-mode should indeed
be sensitive to line numbers, because the header-line in those modes
should align with the rest of buffer text, a counter-example is Info
mode, where indenting the header-line will IMO make the display look
uglier, almost buggy.

If people agree that the effect on header-line should be dependent on
the mode, then it follows that the fix should be in the modes, not in
the display engine.  Fixing tabulated-list-mode will fix all of its
descendants; ruler-mode will need a separate fix.  And likewise for
any other mode which reports a similar issue.