1168921 - box-decoration-break:clone should not clone block margins

Status: RESOLVED FIXED

(Core :: Layout: Block and Inline, defect, P4)

Description

[Mats Palmgren (inactive)]

The CSSWG recently decided that block margins should NOT be cloned
on each fragment, so we should remove that from our implementation
(but only for blocks).

https://lists.w3.org/Archives/Public/www-style/2015May/0279.html

  - RESOLVED: box-decoration-break clones margins (note this only
              affects inlines)

...

box-decoration-break & margins
------------------------------

  fantasai: Last is about box-decoration-break and margins, and
            whether "clone" clones margins.
  fantasai: If I have a box with a border, and it breaks, there's
            two ways to handle it.
  fantasai: First is to just slice the box, so no border at slice.
  fantasai: Other is to wrap the border around fully for each piece.
  fantasai: So question is whether to clone margins too.
  fantasai: So if you had margin-top, does that show up at the top
            of the next fragment?
  Florian: Margin-collapsing?
  fantasai: Interesting.
  [Florian was pointing out that margins at unforced fragmentation
  breaks collapse to nothing.]

  dbaron: And what if multiple collapsing elements have differing
          values for box-decoration-break?
  [This doesn't seem to be an actual problem, each box does what it
      does.]

  fantasai: Current behavior is to restart the next page at the
            border box; no cloning of margins.
  Rossen: Yeah, anything else can get weird, especially with
          negative margins.


box-decoration-break & margins (cont.)
--------------------------------------

  fantasai: And I think issue 13 is that it doesn't matter; margins
            get collapsed into the pagination break.
  Rossen: Yeah.
  dbaron: Margins don't disappear for inlines, right?
  fantasai: Right.
  dbaron: Presumably box-decoration-break applies to inlines, too.
  fantasai: Yes.
  fantasai: So yeah, inlines are still a problem. No strong opinion,
            but since inline margins don't normally collapse into
            the line edge, I think we should keep it (clone inline
            margins)

  RESOLVED: box-decoration-break clones margins (note this only
            affects inlines)

  [Snack break]
  <dbaron> FWIW, Gecko's implementation of
           box-decoration-break:clone on inlines does agree with
           what we just resolved
  <dbaron> i.e., we do clone the margins

Comment 1

[Mats Palmgren (inactive)]

It was implemented in bug 1025669 (part 1).

Comment 2

[Ting-Yu Lin [:TYLin] (PST, UTC-8)]

Attachment: Bug 1168921 - Stop cloning block direction margins for box-decoration-break:clone.

This can also fix bug 1586470.

Comment 3

[Pulsebot]

Pushed by [e-mail]:
https://hg.mozilla.org/integration/autoland/rev/e9783a644016
Stop cloning block direction margins for box-decoration-break:clone. r=mats

Comment 4

[Razvan Maries]

https://hg.mozilla.org/mozilla-central/rev/e9783a644016