Need deeper layout passes inside flexboxes when fragmenting (pagination, multicol).

Need deeper layout passes inside flexboxes when fragmenting (pagination, multicol).

When the fragmentainer (page / column) boundaries have changed, we need to go
deep when laying out. Objects otherwise not marked for layout may still need
layout.

We already do this for regular blocks and when inside tables, but it was
missing for flexbox. Adding this both improves rendering of multicol with
flexbox and gets rid of an assertion failure.

BUG=497981
R=cbiesinger@chromium.org,dsinclair@chromium.org

Committed: https://src.chromium.org/viewvc/blink?view=rev&revision=196795

[dsinclair, 2015-06-09 13:26:41]

This seems fine to me but cbiesinger@ is the flexbox expert so please wait for
his response.

[leviw_travelin_and_unemployed, 2015-06-09 18:43:35]

leviw@chromium.org changed reviewers:
+ leviw@chromium.org

[leviw_travelin_and_unemployed, 2015-06-09 18:43:36]

LGTM.

[cbiesinger, 2015-06-09 19:50:54]

lgtm

So this looks like it makes sense but I don't think I understand why pagination
requires an additional layout pass even if the child size didn't change, can you
explain?

[cbiesinger, 2015-06-09 19:53:17]

oh... this is for the case when the height changes. ok, this makes sense.

[mstensho (USE GERRIT), 2015-06-09 19:58:14]

On 2015/06/09 19:50:54, cbiesinger wrote:
> lgtm
> 
> So this looks like it makes sense but I don't think I understand why
pagination
> requires an additional layout pass even if the child size didn't change, can
you
> explain?

When column height changes, pagination struts may also change. When we have to
push content over to the next column, because it couldn't fit in the current
one, we insert a pagination strut that "eats" the remaining unusable space in
the current column, so that the content that we decided to push to the next
column actually ends up at the top of the next column, by pushing it "downwards"
in the flow thread.

Column balancing may require a few layout passes. If you're interested, you can
take a look at the documentation at the top of
core/layout/LayoutMultiColumnFlowThread.h, which has a section on how we do
column balancing.

Note that this CL doesn't add any more layout passes, though. It just goes
deeper into the tree than it otherwise would.

[mstensho (USE GERRIT), 2015-06-09 19:58:18]

The CQ bit was checked by mstensho@opera.com

[commit-bot: I haz the power, 2015-06-09 19:58:30]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1167323003/1

[commit-bot: I haz the power, 2015-06-09 20:02:05]

Committed patchset #1 (id:1) as
https://src.chromium.org/viewvc/blink?view=rev&revision=196795