[css-inline-3] leading-trim through to descendant line boxes

[fantasai]

The current definition of leading-trim relies on the concept of “first formatted line”. This was done for two reasons:

• it drills down through unstyled block boxes, which is helpful to the author and absolutely necessary given the way we generate anonymous block boxes
• it correctly aborts on nested formatting contexts, replaced elements, etc.

However, it might be a bit too aggressive, as it also drills through styled block boxes. Consider

  

     
...
     
Whatever normal paragraph.
  

The .warning would probably be a bit surprised if the section trimmed its half-leading when it didn't request such trimming itself.

I think we might want to borrow the logic from margin collapsing, and not drill into boxes with non-zero padding/border. Whether or not the margins could have collapsed seems closely related to whether or not this block’s own content edge is adjacent to its descendant's first/last line box and therefore whether it should be controlling the leading on that line.

[css-meeting-bot]

The CSS Working Group just discussed leading-trim through to descendant line boxes, and agreed to the following:

• RESOLVED: Don't drill through in a way we can block boxes with non-zero padding / border in the block axis

The full IRC log of that discussion

Topic: leading-trim through to descendant line boxes
github: https://github.com//issues/5237
fantasai: leading-trim relies on the concept of "first formatted line", which is needed to deal with anon blocks and also nested blocks
... however it may be a bit too aggressive as it also drills into styled boxes
... see the example in the issue, the leading-trim of the outer box ends up affecting the leading of the "warning" box even though it was already styled
... it doesn't seem like the author would spec
This definitely sounds like something that should match up with margin collapsing rules.
... so I think we should do what margin collapsing does and don't drill into boxes with non-zero border / padding
dbaron: do you want to make it also condition on the margin-collapsing definition or just reuse it?
... I think you should intersect the definition
... because otherwise you'd apply it to some empty lines and it'd be bad
q+
iank_: there's a lot of complications round floats and margin-collapsing, which you may be very careful of
ack faceless2
yeah, it might be better to take the relevant parts of the margin-collapsing definition rather than actually referring to it
ack emilio
q-
ack faceless
emilio: would it be better to say anonymous block boxes inherit this property?
fantasai: I think you may want to apply it to non-anonymous boxes as well, like chapter -> paragraph, specially if those margins are collapsed
florian: another one would be the notes in specs, some of them have paragraphs inside some don't, and you want to treat them equally
fantasai: I think I'll try to spec this and then come back
another fun question is whether there are any interactions with 'clear'
RESOLVED: Don't drill through in a way we can block boxes with non-zero padding / border in the block axis
err
s/in a way we can//

[fantasai]

Proposed definition:

Trim the block-start side of the first formatted line to the corresponding 'text-edge' metric of its root inline box. If there is no such line, or if there is intervening non-zero padding or borders, there is no effect.

Agenda+ to confirm as requested by previous discussion.

[css-meeting-bot]

The CSS Working Group just discussed [css-inline-3] leading-trim through to descendant line boxes, and agreed to the following:

• RESOLVED: Accept the proposed text
• RESOLVED: Rename the property from leading-trim to trim-leading

The full IRC log of that discussion

Topic: [css-inline-3] leading-trim through to descendant line boxes
github: https://github.com//issues/5237
fantasai: WG wanted to reconfirm
https://github.com//issues/5237#issuecomment-673312639
fantasai: I left it as first formatted line and no interveneing non-0 padding or borders. Whatever we take for margin we should take here. This is the current text
s/Whatever/whatever clarifications/
dholbert: one question [missed]
dholbert: If you have an inline level span with a border sounds like that would cause the leading trim to be excldued. I think intent was only block level
fantasai: You're trimming the line's leading to the text of root inline box which is by definition unstyled. We can clarify that
smfr: Are we stuck with name?
fantasai: no
+1 to name change
smfr: Can we cal it trim-leading? I think people will read it as leading trim not leading trim
OK with name change
Rossen_: Any reasons why we shouldn't do that?
We don't usually do verbs, but this is a confusing term in the first place.
plinss: Not opposed but I think it'll have the same problem with trim-leading
fantasai: I'm also open to other terms. That's the first one we came up with
ledding
Rossen_: WE can resolve on the rename. What about proposed definition? Can we resolve on that?
Rossen_: Objections?
RESOLVED: Accept the proposed text
Rossen_: Rename to trim-leading. Objections?
RESOLVED: Rename the property from leading-trim to trim-leading

[AmeliaBR]

On the name question: we don't actually use “leading” to describe space between lines anywhere in CSS, do we? Could this be line-height-trim or something similar?

(I myself initially assumed it was “leading” meaning “trim off the starting bit, above the first line”, not “trimming off the extra lead/metal from your type pieces so you can pack them in more compactly”.)

[zed-vector]

Amelia, 'leading' means (a) the strips of lead added between lines of type to increase the line spacing, and (b) the amount by which the line height is so increased.