Reproduced in 56.0.2924.3 on Linux
Works fine in Firefox and Edge

Looks like the ::before element that's supposed to be sized with it's container isn't getting resized when it's container gets resized.

This has to do with the PaintLayerScrollableArea::FreezeScrollbarsScope mechanism in LayoutBlockFlow::layoutBlockFlow(). We fail to detect the height change, because we trigger a re-layout due to the new scrollbar, and in the second layout pass, height doesn't change, so we won't lay out the absolutely positioned descendants.

I have a patch in the works that refactors the code in layoutBlock() and layoutBlockFlow(), which also happens to fix this bug.

Deleted: tc.html 628 bytes

tc.html 628 bytes View Download

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/9d96126f67850daf3c07586dc09fa334d592529c

commit 9d96126f67850daf3c07586dc09fa334d592529c
Author: mstensho <mstensho@opera.com>
Date: Mon Dec 05 21:31:06 2016

Refactor layoutBlock() and layoutBlockFlow(). Happens to fix bugs.

Move what only needs to be done once into layoutBlock(). Rename
layoutBlockFlow() to layoutChildren(). Establish LayoutState once, and compare
with the actual previous height to properly detect height changes.

This fixes two issues with the PaintLayerScrollableArea::FreezeScrollbarsScope
mechanism. Tests added.

1. We used to push LayoutState for the same object twice when freezing
scrollbars, which confused the fragmentation machinery.

2. We failed to detect height changes when freezing scrollbars, because we were
unable to compare against the original height (we compared against the height
we had when entering the second layout pass, rather than comparing against the
one we had when entering the first layout pass). We might therefore end up
skipping necessary re-layout of absolutely positioned descendants.

BUG= 669039 , 670660 

Review-Url: https://codereview.chromium.org/2553833002
Cr-Commit-Position: refs/heads/master@{#436414}

[add] https://crrev.com/9d96126f67850daf3c07586dc09fa334d592529c/third_party/WebKit/LayoutTests/fast/block/shrink-to-fit-with-height-stretched-abspos-and-auto-scrollbar-sibling.html
[add] https://crrev.com/9d96126f67850daf3c07586dc09fa334d592529c/third_party/WebKit/LayoutTests/fragmentation/auto-scrollbar-shrink-to-fit.html
[modify] https://crrev.com/9d96126f67850daf3c07586dc09fa334d592529c/third_party/WebKit/Source/core/layout/LayoutBlockFlow.cpp
[modify] https://crrev.com/9d96126f67850daf3c07586dc09fa334d592529c/third_party/WebKit/Source/core/layout/LayoutBlockFlow.h

 Issue 686391  has been merged into this issue.