Why is tab unaffected by font whereas space is affected?

[Eli Zaretskii]

> Date: Fri, 17 Apr 2020 16:09:41 +0100
> From: Richard Wordingham via Unicode <unicode at unicode.org>
> 
> In some editors, e.g. Emacs 24.4.2, changing the font does
> significantly change the position.  (For most Emacs modes, one nowadays
> has to specify the character as U+0009 to enter the tab as a character.)

Emacs uses a fixed number of space_width pixels to display a TAB.
Since space_width varies with font, so does the width of a TAB.