Support browser font scaling

[kerryarchibald]

Fixes element-hq/element-web#26669

• sets html font size to 100%, and relative to 100% where baseFontSizeV2 is not == default. This change means EW will now respect browser font sizes and scale text accordingly.
• removes useCustomFontSize setting and input
• reinstates displayFunc on Slider component
• displays font slider values as relative to default ("+2")

Checklist

• Tests written for new code (and old code if feasible)
• Linter and other CI checks pass
• Sign-off given on the changes (see CONTRIBUTING.md)

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Here's what your changelog entry will look like:

✨ Features

• Support browser font scaling (#11972). Fixes Support browser font size settings element-hq/element-web#26669. Contributed by @kerryarchibald.

[richvdh]

looks generally sensible to me, modulo the comments here.

Also, CI is sad and there is a conflict.

[richvdh]

is this still true, particularly given we use a custom formatter? Maybe we can switch to a numeric representation and do a bit less toString and parseInt below?

(It might also be nice to switch it to holding a relative value, to make it closer to the displayed value and what it actually does? But happy for us to declare that out of scope for this PR)

[richvdh]

I note there is a reference on line 487 to "16px". It seems like that needs updating, or at least clarifying?

Generally it might be nice to add a little documentation about what baseFontSizeV2 actually does and how its value relates to fontSize on the root element.

[richvdh]

For a bit DRYer:

Suggested change

	const cssFontSize =
	fontSize === FontWatcher.DEFAULT_SIZE
	? "100%"
	: `calc(100% + ${toPx(fontSize - FontWatcher.DEFAULT_SIZE)})`;
	const relativeSize = fontSize - FontWatcher.DEFAULT_SIZE;
	const cssFontSize =
	relativeSize === 0
	? "100%"
	: `calc(100% + ${toPx(relativeSize})`;

(I'd also be inclined to s/${toPx(relativeSize})/${relativeSize}px, but YMMV. I'd also be tempted to get rid of the conditional and just let it be calc(100% + 0px) when it's the default size, but again YMMV)

[robintown]

We went with a different approach: #12246