header-line-format vs. window fringes

[Juanma Barranquero]

I'm trying to use `header-line-format' to create a header line for a
buffer which contains several columns of fixed width. The intent,
obviously, is for the header line to contain titles that should be
aligned right over each corresponding column (assuming the face used
on the buffer and the one for the header line use the same, or
equivalent, monospaced font).

I don't know how to do that when the window has non-null fringes,
because I know the fringes' widths, but I don't see a way to know the
width of the monospaced font, in pixels. As the header doesn't take
into account the fringes, its content is not character-aligned with
the buffer's content.

Am I missing something?

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