Allow disabling custom font size; fixes #9168

[chrisvanpatten]

Fixes #9168.

This introduces a new option to disable setting custom font sizes via add_theme_support('disable-custom-font-sizes');, mirroring the option in add_theme_support('disable-custom-colors');.

This provides greater control for developers over the editor experience.

When set, the numeric input will be hidden:

[chrisvanpatten]

I have another PR I'm working on this morning that depends on this, so I'm keen to get this merged today if possible ❤️

[tofumatt]

So I think this makes sense and the code seems good (an E2E test ensuring the controls don't appear when the editor/theme disables them this would be really good though), but I have a concern: what happens when a user switches from a theme that supports custom fonts to one that doesn't? I guess things'll break because the theme didn't accommodate them?

If that's okay then 🚢, but I'm not sure.

[chrisvanpatten]

@tofumatt it actually doesn't break — your custom font size is preserved, unless you explicitly change it… once you change it to a pre-defined option, you can't go back to custom (unless you close without saving).

That's because all we're doing here is hiding UI, not actually changing how the block is validated. I think this is the correct move; it's safer and prevents data loss.

[tofumatt]

That answers sounds like 🙂