Bug 265276 – Support CSS math functions on flexible lengths

• Component: CSS
• Status: NEW

[Roman Komarov]

To reproduce, go to https://codepen.io/kizu/pen/RwvyyOg or try using `calc(1fr)` in `grid-template-columns`.

The `calc(1fr)` is treated as invalid, but it should be valid, as per specs (see it for the `<flex>` type):

https://drafts.csswg.org/css-values-4/#calc-type-checking

and as clarified by the spec editors: 

https://github.com/w3c/csswg-drafts/issues/6989

> Definitely a bug; `fr` units should be math'able just fine. That note is, indeed, just observing that `fr` is not a `<length>` so `calc(1px + 1fr)` is invalid, but `calc(1fr / 3)` should be just fine.

Curiously, no other browser did fix this yet. Chromium has a bug open from 2019: https://bugs.chromium.org/p/chromium/issues/detail?id=993136&q=calc%20fr&can=2 and I did not find one for Firefox, so I did open it: https://bugzilla.mozilla.org/show_bug.cgi?id=1866236

[Roman Komarov]

I did open a PR adding WPT tests for this: https://github.com/web-platform-tests/wpt/pull/43322

[Radar WebKit Bug Importer]

<rdar://problem/118975697>

[Tim Nguyen (:ntim)]

Sam, is this easy to do with the calc() rewrite you did?

[Sam Weinig]

I can't speak to why the initial codepen doesn't seem to work as the text in the codepen suggests it should (its not 50%) but calc() and fr units is fully supported. (I just don't understand the grid syntax at all so don't know if its correct.

The WPT test added for this, https://wpt.fyi/results/css/css-grid/grid-definition/grid-support-flexible-lengths-001.html?label=experimental&label=master&aligned, seems to be passing in Safari.

[Roman Komarov]

The current WPT tests do not cover the case I mentioned — my wpt PR with the tests for this is still unmerged: https://github.com/web-platform-tests/wpt/pull/43322