Problem

Flutter has two ways to specify the font weight in a TextStyle: you can pass either a FontWeight value or a FontVariation with variation wght. For example these look like TextStyle(fontWeight: FontWeight.w500) or TextStyle(fontVariations: [FontVariation('wght', 500)]).

The trouble is that, at present, neither of these subsumes the job of the other, and nor do they interact well with each other.

• Setting only fontWeight doesn't work if the font that gets used turns out to offer variable weight. This is a pitfall that even Flutter's own Material implementation falls into: Implement full support for variable fonts #109308 (comment)

• Setting only a wght variation doesn't work, I believe, if the font that gets used turns out not to offer variable weight.

• Setting both a fontWeight and a wght variation can have interacting effects:
  fontWeight affects variable font even with FontVariation #136779
  text: Bold text is sometimes way bolder than intended zulip/zulip-flutter#500
  These effects don't follow any documented pattern, and don't seem desirable.

In general it's impossible for an app to know in advance whether the relevant font will offer variable weight: if it's displaying any kind of user-controlled text, the text may be in a variety of scripts, which may not be covered by the app's primary font and may fall back to a variety of other fonts.

As a result, there doesn't seem to be any existing way for a Flutter app to reliably control the font weight of its text, if any of the fonts it uses offer variable weight.

Proposed solution

(See revised proposal below from @Hixie. The changes to FontWeight are the same, but a wght FontVariation retains a separate role, for backwards compatibility and for use in a particular edge case.)

When fontWeight is specified, interpret it as the exact font weight to use, regardless of whether the font offers variable weight and regardless of whether a wght variation is also present. Consult any wght variation only as a fallback, for compatibility, when fontWeight is absent.

In order to support the same fine-grained range of possible weights currently expressed with a wght FontVariation, add a constructor const FontWeight(this.value) that accepts an int. (The FontWeight type is already a class with a field int value.)

This is the same API that CSS offers: there's just one attribute font-weight, it accepts any number from 1 to 1000, and it works regardless of whether the font ultimately used offers variable weight. This design seems to have worked out well for CSS.

Implementation

I think most of the work to implement this would be in the engine. The framework would pass the engine a single font-weight value from 1 to 1000, rather than both a fontWeight encoded as an int from 0 to 8 and a wght variation from 1 to 1000; the engine then needs to use that single value when it comes time to actually select fonts.

Fundamentally this is the same sort of job the engine is already successfully handling: if the app said FontWeight.w500, and the only otherwise-matching fonts are at weights 400 and 700 (the usual "normal" and "bold" values), then the engine has to somehow pick one of those. What's new is just that the weight might be 517 instead of 500.

For the exact behavior to use with that single weight, one natural model to follow is what CSS specifies for the web. That behavior is described on MDN here and here. It's found in the CSS spec here and at the step beginning "font-weight is matched next" within the font-matching algorithm (beginning a little before this example).

Alternatively, if it turns out there's some relatively easy change that can be made to the existing code to handle this, but it has some other behavior that's different from CSS's, then that's probably fine too.