Able to reproduce this issue on reported version 63.0.3239.84 using Windows 10, Ubuntu 14.04 and Mac 10.13.1 with steps mentioned in comment#0. Installed https://chrome.google.com/webstore/detail/arc-welder/emfinbmielocnlhgmfkkmkngdoccbadn/related andObserved blank app page.

Good Build: 63.0.3206.0
Bad Build: 63.0.3207.0

You are probably looking for a change made after 499565 (known good), but no later than 499566 (first known bad).
CHANGELOG URL:
  https://chromium.googlesource.com/chromium/src/+log/5179da1fed16c97090eb18edc507ada543411c9a..0ebd56da93abf970128804390879624b3dc79ac2

Reviewed-on: https://chromium-review.googlesource.com/648369

Suspecting same from changelog.

@hayato: Please confirm the issue and help in re-assigning if it is not related to your change.

Adding RB-Stable as this is recent regression and observed many bugs with this app. Please remove if not the case.

Thanks!

NOTE: Issue is also seen in 65.0.3298.0 dev.

Since hayato@ is off for rest of the year cc'ing kochi@, This app is intended for ChromeOS not for Desktop platforms.

/deep/ and ::shadow CSS selector combinator and pseudo element are announced
to be deprecated for a long time, and we show deprecation message as well.
So this is not a regression introduced by the CL.

Could you reassign someone who is responsible for the UI implmentation of
this?  I assume the UI is based on WebUI (using HTML+CSS), and using the
deprecated / removed CSS selector.

Assigned to jhorwich@ since he's the only one in the ARC Welder rotation who's not OOO.

Unfortunately nobody is actively working on this app anymore (UI or otherwise), but we are trying to maintain it for now.  The reason this is happening is we're still using an old Polymer 0.5 implementation from a couple years ago, and our plans to refresh and update years ago got shelved due to priority of other things.  Two obvious options:

1) push forward to a newer polymer release (this is a lot of work because of the differences between pre-release polymer and new)

2) roll back polymer-based UI and go back to old non-material UI.  I'll take a look at doing this so we can keep this app basically functional

Removing RBS because this app is not still widely used to justify blocking a release

Issue 797761 has been merged into this issue.

 Issue 798940  has been merged into this issue.

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The following revision refers to this bug:
  https://chrome-internal.googlesource.com/arc/arc/+/29dc79f389654b5c1282f0f0935d6373bec50c40

commit 29dc79f389654b5c1282f0f0935d6373bec50c40
Author: Elijah Taylor <elijahtaylor@google.com>
Date: Thu Feb 01 21:41:44 2018

Updated ARC Welder is pushed to production (54.5021.651.0) with reverted material UI.  It's functional if a little uglier

Issue 767186 has been merged into this issue.
Issue 797549 has been merged into this issue.
Issue 803444 has been merged into this issue.
Issue 808405 has been merged into this issue.

Issue 819950 has been merged into this issue.