[css-values][css-images] Unit algebra with <flex> in stripes() leads to a contradiction

[Loirooriol]

https://drafts.csswg.org/css-images-4/#stripes

A <flex> is evaluated as a fraction of the total width relative to the total sum of <flex> entries in the function, after subtracting the thickness of any non-<flex> entries (flooring the subtraction result at zero). If the sum of <flex> values is less than 1fr, the result of the subtraction is multiplied by the sum’s value before being distributed.

Suppose a total width of 100px, what should happen with stripes(#000 50px, #111 calc(1fr / 1fr * 50px), #222 1fr)?

• 1st stripe is a <length> of 50px
• 2nd stripe is a <length> of calc(1fr / 1fr * 50px) 😓
• 3rd stripe is a <flex> of 1fr

Then,

• If we assume that 1fr is not zero, the 2nd stripe is 50px thick, and so we distribute 100px - 50px - 50px into 1fr, so the third stripe is 0px thick. So the final thicknesses are 50px, 50px, 0px, but this is a contradiction because we assumed that 1fr wasn't zero.
• If we assume that 1fr is zero, the 2nd stripe gets a NaN * px which is censored into 0px at the top-level calculation. So we distribute 100px - 50px - 0px into 1fr, so the 3rd stripe is 50px thick. So the final thicknesses are 50px, 0px, 50px, but this is a contradiction because we assumed that 1fr was zero.

Thus it's a contradiction either way. How to solve it?

[SebastianZ]

How to solve it?

By this note in the definition of <flex>:

<flex> values are not <length>s (nor are they compatible with <length>s, like some <percentage> values), so they cannot be represented in or combined with other unit types in calc() expressions.

Sebastian

[Loirooriol]

That refers to calc(1fr + 1px), which is effectively invalid. I'm talking about something else.

[tabatkins]

Right, whether or not you match a production like <length> only looks at the non-zero entries of its type, and in 2fr / 1fr * 1px the type is {flex: 0, length: 1}, which'll successfully match.

[SebastianZ]

That refers to calc(1fr + 1px), which is effectively invalid.

And so is calc(1fr / 1fr * 50px). The definition basically says that <flex> values cannot be used in calc() expressions.

The contradictory is rather that CSS Values 4 says that <flex> values can be represented in calc().

Also, why is this specific to the stripes() function? Doesn't this issue also apply to Grid then? I.e. grid-template-columns: 50px calc(1fr / 1fr * 50px) 1fr; with a 100px wide element.

and in 2fr / 1fr * 1px the type is {flex: 0, length: 1}, which'll successfully match.

@tabatkins So you suggest the result of the example to be 50px, 50px, 0 due to the two fr units cancelling each other out?

Sebastian

[tabatkins]

The definition basically says that values cannot be used in calc() expressions.

No, I think you're misreading/misunderstanding.

Flex values are not compatible with lengths; you can't add them together. But you can use flex values in an expression with lengths, so long as you never actually combine it with a length value in a way that uses type addition. So 1fr / 1fr * 1px is completely valid (final type {flex: 0, length: 1}, matches <length>), but 1fr + 1px is invalid (type addition notices each has non-zero types that aren't present in the other, so fails).

So you suggest the result of the example to be 50px, 50px, 0 due to the two fr units cancelling each other out?

That is what I'd naively expect, and it might be reasonable to fix the spec to allow it, yes. I haven't been able to fully think it thru yet, tho.

[Loirooriol]

I have been thinking further about this, and I believe I was mistaken, because:

• Whether 1fr ends up having the same behavior as 0px is irrelevant, <flex> is not being resolved into a length so we aren't actually dividing by zero
• Only a top-level <flex> in a <color-stripe> ends up behaving as some length. These are just intermediate values so they don't obey https://drafts.csswg.org/css-images-4/#valdef-stripes-flex