[css-overflow] How do empty blocks affect line-clamping by height?

[andreubotella]

<div style="line-clamp: auto; max-height: 2lh">
  <!-- Possible clamp point A -->
  <div>Line 1</div>
  <!-- Possible clamp point B -->
  <div>Line 2</div>
  <!-- Possible clamp point C -->
  <div></div> <!-- Takes up zero height -->
  <!-- Possible clamp point D -->
  <div>Line 3</div>
</div>

The way line-clamp is currently defined, the last possible clamp point that doesn't overflow would be chosen, and since the div between C and D takes up zero height, we'd clamp at clamp point D. By the rules of block-ellipsis, then this would mean that line 2 would not be ellipsized, since there are more things other than absolutely positioned elements and end-of-elements between the line box and the clamp point. Should this be the case?

This is similar to the discussion about abspos in #10868 (comment), with the difference that an abspos has no rendering differences whether the clamp point is before or after it, since abspos will render unless their containing block is completely after the clamp point (see #11379 and #11962). But this is not true for zero-height blocks:

• A zero-height block could be the containing block for an abspos, meaning that the abspos will be shown or hidden depending on whether the clamp point is before or after the CB.
• A block could be explicitly set to have height: 0 while having content that visibly overflows.

So, should empty / zero-height blocks be treated specially when choosing a clamp point, or when determining whether a line box should be ellipsized?

[css-meeting-bot]

The CSS Working Group just discussed [css-overflow] How do empty blocks affect line-clamping by height?, and agreed to the following:

• RESOLVED: Close no change until/unless users complain.

The full IRC log of that discussion

<fantasai> andreubotella: Split off from a previous discussion on abspos and stuff.
<fantasai> andreubotella: When clamping by height, the last possible clamp point that doesn't overflow is chosen.
<fantasai> andreubotella: If you have a block element that has no content, it would clamp after it.
<fantasai> andreubotella: That's probably fine, except it means that if there's a line before that empty block, it will not be ellipsized.
<fantasai> andreubotella: The presence or absence of that block before/after clamp point, if that block is empty it doesn't matter. But if the block has position:relative, and contains an abspos, it does matter because it affects whether abspos is hidden or shown
<fantasai> andreubotella: this can also affect boxes that have non-zero content, but height set to zero
<fantasai> andreubotella: In those cases, the content of the box would also be shown or not shown depending.
<fantasai> andreubotella: I think it makes sense to clamp after zero-height boxes. Find the last possible clamp point before the height overflows
<fantasai> florian: Main downside is because last thing before clamp point isn't the line, we don't see the ellipsis
<astearns> ack fantasai
<fantasai> florian: Although in case of zero height *and* content, ellipsis would be seen on that content.
<fantasai> florian: but I guess it's ok, don't put empty <div>s where you don't need them.
<astearns> q+
<fantasai> andreubotella: If you have a box which is zero-height, but does have content (whether overflowing or abspos), you probably don't want the ellipsis to be placed on the previous line
<astearns> q-
<fantasai> andreubotella: should we depend on that?
<astearns> fantasai: if you have a para and then a block that’s a horiz rule
<astearns> fantasai: do you want that para to be ellipsised?
<fantasai> andreubotella: Currently it's not
<fantasai> andreubotella: block-ellipsis applies to the last line before clamp point if only thing in between are abspos or end of element
<astearns> fantasai: is that the behavior you want?
<fantasai> florian: We discussed earlier, and resolved on this behavior given no perfect solution.
<fantasai> andreubotella: For some use cases you might want it, and for others you might not.
<fantasai> andreubotella: [missed example]
<fantasai> astearns: So proposed resolution is no special handling for zero-height or empty blocks?
<andreubotella> s/[missed example]/if you have a line, and then an independent formatting context, you don't want the line before it to have the ellipsis/
<astearns> fantasai: could we just say the kinds of blocks that inhibit margin collapsing…
<fantasai> florian: Are you saying we should lean more on margin-collapsing-through??
<fantasai> astearns: Could close no change and then explore later
<fantasai> florian: It's not interaction with margin-collapsing, it's trying to identify which blocks we consider and those we don't
<fantasai> fantasai: No special handling, if people complain we'll reconsider
<fantasai> PROPOSED: Close no change until/unless users complain.
<fantasai> RESOLVED: Close no change until/unless users complain.

[frivoal]

tested by web-platform-tests/wpt#55002 and web-platform-tests/wpt#54999