Tested on Canary #62.0.3167.0 on Mac 10.12.5 and able to reproduce the issue. Issue is also observed on Windows 7 Canary #62.0.3166.0 and Ubuntu 14.04 latest Dev #61.0.3159.5

This is a non-regression issue and able to reproduce from M-61 #61.0.3149.0. Marking it as untriaged so that issue gets addressed.

Note: For the builds earlier to 61.0.3149.0, provided html file displays a blank screen.

Thanks.

> Note: For the builds earlier to 61.0.3149.0, provided html file displays a blank screen.

Here's a modified version of the repro page without module scripts; it exhibits the same issue in stable (59.0.3071.115).

Deleted: slotted-selector.html 2.2 KB

slotted-selector.html 2.2 KB View Download

kochi@, do you have any insights?

I'll investigate.

This was because the style sheet for each shadow dom are categorized
as "containing tree-boundary rules" and matched against all other tree
scopes - which resulted in O(n^2) behavior.

The fix CL is in progress.

The following revision refers to this bug:
  https://chromium.googlesource.com/chromium/src.git/+/ab70dc71b6156e8e3b79310463f668d96b842796

commit ab70dc71b6156e8e3b79310463f668d96b842796
Author: Takayoshi Kochi <kochi@chromium.org>
Date: Mon Aug 07 08:02:59 2017

Decouple ::slotted rules from TreeBoundaryCrossing rules

::slotted() pseudo element rules were treated as tree-boundary
crossing rules (same as legacy /deep/ and ::shadow).
This caused O(N^2) issue where N is the node of shadow trees with
::slotted() rules, but without real tree-boundary crossing rules,
as adding the scope to StyleEngine's tree boundary crossing scopes
is slow.

The perf issue of rule matching was already fixed by Rune's old CL:
https://codereview.chromium.org/1853713002/

This CL is a performance fix and should not expose any behavioral
changes except for performance.

With the streamlined test case from the original report (N=100,1000)
http://jsbin.com/nodiwuxuge/edit?html,console,output
I got the following results on my local workstation:

Without this fix: 15.7ms (N=100), 1495ms (N=1000)
With this fix: 14.9ms (N=100), 93.4ms (N=1000)

Bug:  748790 
Change-Id: I4b3c4b29fcdd4bf152ed3e6409044df748eedc93
Reviewed-on: https://chromium-review.googlesource.com/595295
Reviewed-by: Rune Lillesveen <rune@opera.com>
Commit-Queue: Takayoshi Kochi <kochi@chromium.org>
Cr-Commit-Position: refs/heads/master@{#492282}
[add] https://crrev.com/ab70dc71b6156e8e3b79310463f668d96b842796/third_party/WebKit/LayoutTests/shadow-dom/v0/slotted-pseudo-element-in-v0v1-mixed.html
[modify] https://crrev.com/ab70dc71b6156e8e3b79310463f668d96b842796/third_party/WebKit/Source/core/css/ActiveStyleSheetsTest.cpp
[modify] https://crrev.com/ab70dc71b6156e8e3b79310463f668d96b842796/third_party/WebKit/Source/core/css/resolver/ScopedStyleResolver.cpp
[modify] https://crrev.com/ab70dc71b6156e8e3b79310463f668d96b842796/third_party/WebKit/Source/core/css/resolver/ScopedStyleResolver.h
[modify] https://crrev.com/ab70dc71b6156e8e3b79310463f668d96b842796/third_party/WebKit/Source/core/css/resolver/StyleResolver.cpp