Why is tab unaffected by font whereas space is affected?

[Kent Karlsson]

> 20 apr. 2020 kl. 22:48 skrev Asmus Freytag via Unicode <unicode at unicode.org>:
> 
> On 4/20/2020 7:36 AM, Eli Zaretskii via Unicode wrote:
>>> In summary, tab stops are at particular positions in a displayed line of text, and do not depend on
>>> font changes, or font size changes. In some contexts one can set the tab stops (not just using
>>> default positions), and they ”stay” over font changes and font size changes, until tab stops are
>>> (re)set by a tab setting ”command” (or paragraph property, or similar mechanism, depending
>>> on system).
>> That would mean, for example, that if you make the font smaller, the
>> tab stops stay in the same positions, pixel-wise?  isn't that strange?
>> 
> No, that's how every word processor and text layout system has worked from day one (except apparently programming editors).
> 
> On typewriters, if your machine had adjustable tabs, you could change your mind about tab stop positions at any time while in the middle of a line.
> 
For a manual (non-electric) typewriter I once had, that was true. (I did not keep it, which I now regret…)

For the Diablo typewriter terminal (Diablo 1620), yes I have used one, it had no tab stops set at power-up (it then moved the print position to the ”far right” when outputting a tab character). One had to explicitly set tab stops (with proprietary escape sequences) to get any tab stops. So I would agree that having some default tab stops is handy...
> But generally, in rich text environment, these are properties of blocks of text (paragraphs) and don't track with font size.
> 
Indeed (in principle, not so sure tab stops need be bundled with paragraph properties). But one can say like this: 
changing the *default* font (typeface and/or em size), as a preference setting that apply ”globally” (i.e. not just to a text ”run” (substring), for some notion of ”globally"), if the application allows for such a preference setting:
that may imply a change of default tab stops (and some applications might have only the default tab stops, not any explicitly set ones).

So in this case one can eat the cake and have it too… Changing the default font *preference* (implicitly changing default tab stops ”globally”, but not changing any explicitly set tab stops) is different from changing the font for a text *run* (which would never change any tab stops, whether default or explicit). (For emacs, there would only be the (currently set) default font, and (the currently set) default tab stops. A ”word processor” may or may not allow user setting of the default font as a preference.)

/Kent K

> A./
> 

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