Don't clamp scroll positions until the end of all layouts

Make sure to save/restore scroll positions in finishDelayUpdateScrollInfo

Due to Flexbox's multi-pass layout, we may lay out children at
intermediate sizes that are very small, causing us to clamp the
scroll position at this small size. Once we lay out at the right
size, the scroll position is lost.

To avoid this problem, make finishDelayUpdateScrollInfo store
the old scroll position so it can be restored.

XXXXX THIS IS TEMPORARY XXXXX

The plan is to bake this on canary and merge this to M50 to
fix the release blocker and fix this the right way on trunk
(bug 600036), by delaying clamping of scroll offsets to after layout.

BUG=593209
Committed: https://crrev.com/ea799d37bd27d7ae346031a9332f542f7f57fd38
Cr-Commit-Position: refs/heads/master@{#384772}

[cbiesinger, 2016-04-01 02:12:01]

Description was changed from

==========
fix scrolling

BUG=
==========

to

==========
Make sure to save/restore scroll positions in finishDelayUpdateScrollInfo

Due to Flexbox's multi-pass layout, we may lay out children at
intermediate sizes that are very small, causing us to clamp the
scroll position at this small size. Once we lay out at the right
size, the scroll position is lost.

To avoid this problem, make finishDelayUpdateScrollInfo store
the old scroll position so it can be restored.

BUG=593209
==========

[cbiesinger, 2016-04-01 03:11:21]

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

[cbiesinger, 2016-04-01 03:11:39]

The CQ bit was checked by cbiesinger@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-04-01 03:12:26]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1846023003/20001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1846023003/20001

[commit-bot: I haz the power, 2016-04-01 04:21:54]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-04-01 04:21:55]

Dry run: This issue passed the CQ dry run.

[leviw_travelin_and_unemployed, 2016-04-01 18:05:10]

https://codereview.chromium.org/1846023003/diff/20001/third_party/WebKit/Sour...
File third_party/WebKit/Source/core/layout/LayoutFlexibleBox.cpp (right):

https://codereview.chromium.org/1846023003/diff/20001/third_party/WebKit/Sour...
third_party/WebKit/Source/core/layout/LayoutFlexibleBox.cpp:292: if
(LayoutBlock::finishDelayUpdateScrollInfo(&layoutScope, &scrollMap)) {
The lack of a test makes me a sad panda.

It seems like we're only storing one level deep here. If the flexboxes are
nested, the first pass of the inner flexbox, if smaller, will still lose its
position. Does that make sense? 

Why doesn't the map need the same lifetime as gDelayUpdateScrollInfo (storing
the initial values in the first pass and updating them only when we're
finished)?

[cbiesinger, 2016-04-01 18:45:32]

Description was changed from

==========
Make sure to save/restore scroll positions in finishDelayUpdateScrollInfo

Due to Flexbox's multi-pass layout, we may lay out children at
intermediate sizes that are very small, causing us to clamp the
scroll position at this small size. Once we lay out at the right
size, the scroll position is lost.

To avoid this problem, make finishDelayUpdateScrollInfo store
the old scroll position so it can be restored.

BUG=593209
==========

to

==========
Don't clamp scroll positions until the end of all layouts

Due to Flexbox's multi-pass layout, we may lay out children at
intermediate sizes that are very small, causing us to clamp the
scroll position at this small size. Once we lay out at the right
size, the scroll position is lost.

To avoid this problem, move the clamping to the post-layout pass
through paint layers via updateLayerPositionsAfterLayout.
BUG=593209
==========

[cbiesinger, 2016-04-01 18:45:32]

cbiesinger@chromium.org changed reviewers:
+ esprehn@chromium.org

[cbiesinger, 2016-04-01 18:45:46]

Description was changed from

==========
Don't clamp scroll positions until the end of all layouts

Due to Flexbox's multi-pass layout, we may lay out children at
intermediate sizes that are very small, causing us to clamp the
scroll position at this small size. Once we lay out at the right
size, the scroll position is lost.

To avoid this problem, move the clamping to the post-layout pass
through paint layers via updateLayerPositionsAfterLayout.
BUG=593209
==========

to

==========
Don't clamp scroll positions until the end of all layouts

Due to Flexbox's multi-pass layout, we may lay out children at
intermediate sizes that are very small, causing us to clamp the
scroll position at this small size. Once we lay out at the right
size, the scroll position is lost.

To avoid this problem, move the clamping to the post-layout pass
through paint layers via updateLayerPositionsAfterLayout.

BUG=593209
==========

[cbiesinger, 2016-04-01 18:46:06]

The CQ bit was checked by cbiesinger@chromium.org to run a CQ dry run

[commit-bot: I haz the power, 2016-04-01 18:46:33]

Dry run: CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1846023003/40001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1846023003/40001

[cbiesinger, 2016-04-01 18:46:34]

Now with a much better approach, see the new CL description.

Still working on getting a testcase, it's hard.

[leviw_travelin_and_unemployed, 2016-04-01 19:14:39]

I'm hoping for a test case, but I like this approach a lot more :)

[commit-bot: I haz the power, 2016-04-01 19:46:58]

The CQ bit was unchecked by commit-bot@chromium.org

[commit-bot: I haz the power, 2016-04-01 19:46:59]

Dry run: Try jobs failed on following builders:
  win_chromium_rel_ng on tryserver.chromium.win (JOB_FAILED,
http://build.chromium.org/p/tryserver.chromium.win/builders/win_chromium_rel_...)

[cbiesinger, 2016-04-02 00:08:16]

I'm going back to the working approach so we have something to merge to M50, as
this is a release blocker.

I filed a bug to fix this the right way, see comment in patch.

[cbiesinger, 2016-04-02 00:09:09]

Description was changed from

==========
Don't clamp scroll positions until the end of all layouts

Due to Flexbox's multi-pass layout, we may lay out children at
intermediate sizes that are very small, causing us to clamp the
scroll position at this small size. Once we lay out at the right
size, the scroll position is lost.

To avoid this problem, move the clamping to the post-layout pass
through paint layers via updateLayerPositionsAfterLayout.

BUG=593209
==========

to

==========
Make sure to save/restore scroll positions in finishDelayUpdateScrollInfo

Due to Flexbox's multi-pass layout, we may lay out children at
intermediate sizes that are very small, causing us to clamp the
scroll position at this small size. Once we lay out at the right
size, the scroll position is lost.

To avoid this problem, make finishDelayUpdateScrollInfo store
the old scroll position so it can be restored.

BUG=593209
==========

[leviw_travelin_and_unemployed, 2016-04-02 00:09:47]

Can you update the description describing the temporary nature of this fix?
Mention that this is just meant to bake in Canary to ensure it's not going to
cause more issues, then we can merge it and revert it :)

LGTM for this as a temporary solution.

[cbiesinger, 2016-04-02 00:12:39]

Description was changed from

==========
Make sure to save/restore scroll positions in finishDelayUpdateScrollInfo

Due to Flexbox's multi-pass layout, we may lay out children at
intermediate sizes that are very small, causing us to clamp the
scroll position at this small size. Once we lay out at the right
size, the scroll position is lost.

To avoid this problem, make finishDelayUpdateScrollInfo store
the old scroll position so it can be restored.

BUG=593209
==========

to

==========
Make sure to save/restore scroll positions in finishDelayUpdateScrollInfo

Due to Flexbox's multi-pass layout, we may lay out children at
intermediate sizes that are very small, causing us to clamp the
scroll position at this small size. Once we lay out at the right
size, the scroll position is lost.

To avoid this problem, make finishDelayUpdateScrollInfo store
the old scroll position so it can be restored.

XXXXX THIS IS TEMPORARY XXXXX

The plan is to bake this on canary and merge this to M50 to
fix the release blocker and fix this the right way on trunk
(bug 600036), by delaying clamping of scroll offsets to after layout.

BUG=593209
==========

[cbiesinger, 2016-04-02 00:12:45]

The CQ bit was checked by cbiesinger@chromium.org

[commit-bot: I haz the power, 2016-04-02 00:13:22]

CQ is trying da patch. Follow status at
 https://chromium-cq-status.appspot.com/patch-status/1846023003/80001
View timeline at
 https://chromium-cq-status.appspot.com/patch-timeline/1846023003/80001

[commit-bot: I haz the power, 2016-04-02 01:28:34]

Description was changed from

==========
Make sure to save/restore scroll positions in finishDelayUpdateScrollInfo

Due to Flexbox's multi-pass layout, we may lay out children at
intermediate sizes that are very small, causing us to clamp the
scroll position at this small size. Once we lay out at the right
size, the scroll position is lost.

To avoid this problem, make finishDelayUpdateScrollInfo store
the old scroll position so it can be restored.

XXXXX THIS IS TEMPORARY XXXXX

The plan is to bake this on canary and merge this to M50 to
fix the release blocker and fix this the right way on trunk
(bug 600036), by delaying clamping of scroll offsets to after layout.

BUG=593209
==========

to

==========
Make sure to save/restore scroll positions in finishDelayUpdateScrollInfo

Due to Flexbox's multi-pass layout, we may lay out children at
intermediate sizes that are very small, causing us to clamp the
scroll position at this small size. Once we lay out at the right
size, the scroll position is lost.

To avoid this problem, make finishDelayUpdateScrollInfo store
the old scroll position so it can be restored.

XXXXX THIS IS TEMPORARY XXXXX

The plan is to bake this on canary and merge this to M50 to
fix the release blocker and fix this the right way on trunk
(bug 600036), by delaying clamping of scroll offsets to after layout.

BUG=593209
==========

[commit-bot: I haz the power, 2016-04-02 01:28:35]

Committed patchset #5 (id:80001)

[commit-bot: I haz the power, 2016-04-02 01:30:10]

Description was changed from

==========
Make sure to save/restore scroll positions in finishDelayUpdateScrollInfo

Due to Flexbox's multi-pass layout, we may lay out children at
intermediate sizes that are very small, causing us to clamp the
scroll position at this small size. Once we lay out at the right
size, the scroll position is lost.

To avoid this problem, make finishDelayUpdateScrollInfo store
the old scroll position so it can be restored.

XXXXX THIS IS TEMPORARY XXXXX

The plan is to bake this on canary and merge this to M50 to
fix the release blocker and fix this the right way on trunk
(bug 600036), by delaying clamping of scroll offsets to after layout.

BUG=593209
==========

to

==========
Make sure to save/restore scroll positions in finishDelayUpdateScrollInfo

Due to Flexbox's multi-pass layout, we may lay out children at
intermediate sizes that are very small, causing us to clamp the
scroll position at this small size. Once we lay out at the right
size, the scroll position is lost.

To avoid this problem, make finishDelayUpdateScrollInfo store
the old scroll position so it can be restored.

XXXXX THIS IS TEMPORARY XXXXX

The plan is to bake this on canary and merge this to M50 to
fix the release blocker and fix this the right way on trunk
(bug 600036), by delaying clamping of scroll offsets to after layout.

BUG=593209

Committed: https://crrev.com/ea799d37bd27d7ae346031a9332f542f7f57fd38
Cr-Commit-Position: refs/heads/master@{#384772}
==========

[commit-bot: I haz the power, 2016-04-02 01:30:12]

Patchset 5 (id:??) landed as
https://crrev.com/ea799d37bd27d7ae346031a9332f542f7f57fd38
Cr-Commit-Position: refs/heads/master@{#384772}