Note: Issue is not seen in Mac and Linux OS.

Adding RB label as this is a recent regression

** IMPORTANT change in M52 merge date due to first 2 weeks of July no release weeks **
M52 Stable is launching very soon! Your bug is labelled as Stable ReleaseBlock, pls make sure to land the fix and get it merged ASAP. All changes MUST be merged into the release branch by 5pm on July 1 to make into the desktop Stable final build cut. Thank you!

This is an old overflow recalc bug exposed by my CL. The problem is in LayoutBlock::recalcChildOverflowAfterStyleChange() when iterating the positioned children. The positionedChildren() is only updated after laying out children, so during recalc overflow we are using stale information in it.

We should either mark the object needs (simplified) layout when it becomes out-of-flow-positioned or non-(out-of-flow-positioned), or we should update positionedChildren() during or after style change, but that requires correct layout tree insertion/deletion order.

https://codereview.chromium.org/2103843003/

Update containing block's positioned descendants list when an object becomes out-of-flow-positioned

This is required when there are descendants needing overflow recalc.
Otherwise during overflow recalc because the containing block's
positioned descendant list didn't contain the new out-of-flow-positioned
object we would not reach the descendants needing overflow recalc.

BUG=623867 <== The bug number was wrong. Should be 622655 (this bug).
TEST=fast/overflow/add-visual-overflow-and-change-container-position.html

Committed: https://crrev.com/fd08a658d68598dce7886abf88fbc71ed6659f41
Cr-Commit-Position: refs/heads/master@{#402550}

Your change meets the bar and is auto-approved for M52 (branch: 2743)

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ea9c8c0ffffe09bb297ed9c5c619f411e06f7250

commit ea9c8c0ffffe09bb297ed9c5c619f411e06f7250
Author: Xianzhu Wang <wangxianzhu@chromium.org>
Date: Wed Jun 29 20:53:08 2016

Update containing block's positioned descendants list when an object becomes out-of-flow-positioned

This is required when there are descendants needing overflow recalc.
Otherwise during overflow recalc because the containing block's
positioned descendant list didn't contain the new out-of-flow-positioned
object we would not reach the descendants needing overflow recalc.

BUG= 623867 
TEST=fast/overflow/add-visual-overflow-and-change-container-position.html

Review URL: https://codereview.chromium.org/2111643002 .

Review-Url: https://codereview.chromium.org/2103843003
Cr-Original-Commit-Position: refs/heads/master@{#402550}
Cr-Commit-Position: refs/branch-heads/2743@{#531}
Cr-Branched-From: 2b3ae3b8090361f8af5a611712fc1a5ab2de53cb-refs/heads/master@{#394939}

[add] https://crrev.com/ea9c8c0ffffe09bb297ed9c5c619f411e06f7250/third_party/WebKit/LayoutTests/fast/overflow/add-visual-overflow-and-change-container-position-expected.html
[add] https://crrev.com/ea9c8c0ffffe09bb297ed9c5c619f411e06f7250/third_party/WebKit/LayoutTests/fast/overflow/add-visual-overflow-and-change-container-position.html
[modify] https://crrev.com/ea9c8c0ffffe09bb297ed9c5c619f411e06f7250/third_party/WebKit/Source/core/layout/LayoutBlock.cpp

The fix is working fine on Win7/64 bit, Mac OSX 10.11.5 & Linux/Ubuntu 14.04 - Version 52.0.2743.60 (64-bit) Beta build.

No gray line is observed while scrolling the page.  

wangxianzhu@, can you please remove the Merge-Approved-52 label if nothing is pending on this bug ?