bug#55387: 28.1.50; csv-mode: header does not align when line numbers are displayed

[Lars Ingebrigtsen]

On the other hand, factoring out the code in tabulated-list.el into its
own little minor mode -- say, `header-line-indent-mode' -- looks like it
would be pretty easy, and not involve any complicated state, too.  (It
just runs off of pre-redisplay-functions/window-scroll-functions.)

As a bonus it doesn't require that that modes change their
header-line-format variables, I think.

I'll take a stab at doing that.

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   bloggy blog: http://lars.ingebrigtsen.no