[css-inline-3] Apply leading-trim to inlines?

[fantasai]

We added the ability to trim not just leading, but also space within the glyph to various metrics for the first/last line boxes of a block container. Do we need this ability for inlines, too? It might help with avoiding various magic numbers when setting padding on inlines with backgrounds or borders.

[css-meeting-bot]

The CSS Working Group just discussed Apply leading-trim to inlines? #3955, and agreed to the following:

• RESOLVED: leading-trim applies to inlines by changing the content box edges

The full IRC log of that discussion

<fantasai> Topic: Apply leading-trim to inlines? #3955
<RossenF2F> github: https://github.com//issues/3955
<emilio> fantasai: the purpose of leading-trim is to change the bottom edge of a block so that you can make more useful alignment of block
<emilio> ... may be useful to make it apply to inlines
<emilio> ... so that you can change the edge of the background boxes of inlines
<emilio> ... right now people try to find padding values to make text look centered
<emilio> ... this property allows to control the edge of blocks, but we could make it apply to inlines too
<emilio> myles: I think it was a mistake to make line-height apply to inlines and this feels similar
<emilio> fantasai: this doesn't really need to affect layout
<emilio> ... right now when you add padding / borders to an inline it doesn't affect the position of the inline
<RossenF2F> ack dbaron
<emilio> dbaron: feels like making this apply to inlines make people use backgrounds on inlines reliably, so it seems good to me
<florian> +1
<emilio> ... particularly noting that it doesn't affect the line-height calculations, only the content-box
<emilio> koji: this proposal looks like we're trying to change text-top based on something which sounds like a circular dependency
<emilio> ... with leading-trim
<emilio> fantasai: well not really this allows to override the content edge of backgrounds, I don't think it's inconsistent or circular
<emilio> koji: so leading-trim: cap will only affect the background?
<emilio> fantasai: if set on a block it sets changes the position of the line
<emilio> ... but I propose that when you're using it on an inline it only changes the edge of the background
<RossenF2F> q?
<emilio> [[ blackboard time ]]
<emilio> koji: I want to ship the block part first so I'd prefer this to be a separate property
<emilio> ... so that authors can detect support for it with @support
<emilio> fantasai: we have the same issue with alignment properties
<emilio> ... it's not great
<emilio> dbaron: I think it's a little risky to do that
<RossenF2F> q?
<fantasai> s/that/that, but it's allowed/
<myles> s/I think it was a mistake to make line-height apply to inlines and this feels similar//
<emilio> ... but I think once you have it working for one type it shouldn't be a lot of work to have it working for inlines too
<RossenF2F> Image of blackboard at https://lists.w3.org/Archives/Public/www-archive/2020Jan/0009.html
<emilio> koji: if the WG is fine with use shipping the block part first that's fine for me
<emilio> fantasai: yeah seems better to have an awkward transition than needing a property per box type
<emilio> koji: it's a bit risky...
<emilio> fantasai: backgrounds on inlines are fairly uncommon
<emilio> iank_: I don't think you could detect this via script
<emilio> fantasai: I think it'd change getBoundingClientRecT()
<emilio> ... it wouldn't change layout because the layout of an inline doesn't depend on the content area
<emilio> ... but you can observe it via script
<emilio> RESOLVED: leading-trim applies to inlines by changing the content box edges