[css-align-3] safe vs unsafe defaults for block containers

[fantasai]

I'd like to propose making the default behavior of align-content be safe rather than unsafe for block containers that are not scroll containers.

I think this is a better default for block containers, and is likely also safer for Web-compat if we want to actually deploy this.

[dholbert]

I'd like to propose making the default behavior of align-content be safe rather than unsafe for block containers that are not scroll containers.

Why the "that are not scroll containers" limitation? (I'm not necessarily objecting to it; I just want to understand the reasoning.)

At first glance, it seems like scroll containers may be a bit more risky since they impose a local clip on any unsafely-overflowing content that overflow off their start edge. (At least, that's how align-content:center; justify-content:center behave with overflow:scroll flex containers right now, in https://jsfiddle.net/58eg3a4w/ )

[dholbert]

(posting the answer to my question after chatting with @fantasai in person): Per spec, scroll containers aren't supposed to clip stuff that gets pushed off the start edges via *-content. They're just supposed to establish a different initial scroll position.

That's not implemented anywhere yet, but it's what the spec says is supposed to happen, and it's why this in theory isn't an issue for blocks-that-are-scroll-containers.

[css-meeting-bot]

The CSS Working Group just discussed [css-align-3] safe vs unsafe defaults for block containers, and agreed to the following:

• RESOLVED: default behavior for align-content on block containers is safe

The full IRC log of that discussion

<emilio> fantasai: currently when you apply align-/justify-content to flex/grid they are unsafe, but ua's are not supposed to shift stuff off the unscrollable region, but nobody implements that
<emilio> ... for safe centering if the content is more than the container you clip to the top
<emilio> ... the spec behavior is technically limit by the scroll-container edges but nobody implements that
<emilio> ... the align is spec'd to also apply to block containers
<emilio> ... currently nobody implements that except the Firefox checkout on my laptop
<emilio> ... there are some concerns about web compat but also whether unsafe centering is right for block
<bramus> q+
<emilio> ... I think for block layout in general safe centering by default makes more sense
<emilio> ... also reduces the chances of a compat issue
<emilio> ... proposal is that for block containers we default to safe alignment
<emilio> Rossen_: this would be in both directions right?
<emilio> fantasai: block containers don't support justify-content, just align-content
<iank_> q+
<Rossen_> ack bramus
<emilio> bramus: the default grid/flex has definitely tripped off authors
<emilio> ... so +1 but people might rely on unsafe alignment?
<emilio> fantasai: not implemented anywhere yet
<Rossen_> ack iank_
<emilio> iank_: this seems fine overall
<oriol> q+
<emilio> ... we find that for table cells where they fall back to vertical-alignment: middle it uses unsafe
<emilio> ... as long as it's not messing with table cells seems fine
<emilio> ... I don't think it's a huge compat mitigation, the biggest risk is the new bfc behavior
<emilio> oriol: I'm reading that you're proposing it for block containers that are not scroll-container
<emilio> ... do you want to keep this distinction? If so, why?
<emilio> fantasai: for block containers that are scroll containers you don't have this unsafeness because you don't have overflow
<emilio> ... your entire scrollable region is aligned and the initial scroll position changes
<emilio> ... per spec, you're not supposed to shift the content
<emilio> ... you're supposed to change the initial scroll position
<emilio> oriol: I also wanted to ask that you mentioned that the default is unsafe, but wasn't there some magic if neither was specified?
<emilio> fantasai: yeah, which nobody implements. we wouldn't do that for blocks and just default to safe which is also simplere
<emilio> s/simplere/simpler
<emilio> RESOLVED: default behavior for align-content on block containers is safe

[yisibl]

Is there a WPT test here?