Thanks for filing the issue!

Unable to reproduce the issue on reported chrome version 64.0.3282.186 using Ubuntu 14.04 with the below mentioned steps.
1. Launched Chrome
2. Downloaded demo.html file
3. Opened the downloaded file
We did not observe any rectangle in the circle. Attaching the screen cast of the same.

@Reporter: Could you please have a look at the screen cast and let us know if anything missed from our end. Any further inputs from your end may help us.

Deleted: 820809.mp4 835 KB

	820809.mp4 835 KB View Download

OS: Ubuntu 16.04.4 LTS

Disabling "3D software rasterizer" in chrome://flags solves the problem.

Attached chrome://gpu dump and video reproduction

Deleted: chrome_gpu 27.9 KB

chrome_gpu 27.9 KB View Download

Deleted: chrome_bug.mp4 75.5 KB

	chrome_bug.mp4 75.5 KB View Download

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Able to reproduce the issue on reported chrome version 64.0.3282.186 and on the latest dev 67.0.3368.0 using Ubuntu 17.10.
Note: Issue is not seen on Mac 10.13.1 and Windows 10

Bisect Information:
====================
Last Good Build: 62.0.3175.0
First Bad Build: 62.0.3176.0

You are probably looking for a change made after 491731 (known good), but no later than 491732 (first known bad).
CHANGELOG URL:
  https://chromium.googlesource.com/chromium/src/+log/18873a37dea920be335c7ede66525b597d251a46..a5f7079f3c63aa9cec376886fddb1141527bfd53
Suspecting the same from the above CL.
Review URL: https://chromium-review.googlesource.com/591913

@sunxd: Please help in assigning it to others, if this is not related to your change.

Thanks!

Looks to me that the opacity is not re-rastered in time for the animation on the center solid color quad of the border-radius element and it happens with newer version of ubuntu.

The issue does not reproduce on 14.04 LTS and I'm borrowing such a machine for debug.

According to smcgruer@, the issue does not reproduce if run with --disable-gpu flag, so I guess it has something to do with gl logic.

The attached recording with made without --disable-gpu, on the following setup:

Google Chrome 65.0.3325.162 (Official Build) (64-bit)

#140~14.04.1-Ubuntu SMP Fri Feb 16 09:25:20 UTC 2018

Deleted: out.ogv 3.6 MB

	out.ogv 3.6 MB View Download

This is a P1 Bug-Regression that has not had an update since March 13th. sunxd@, can you confirm if you are intending to work on this item, or if it has been mis-prioritized?

I'll make it P2 and investigate this after site isolation release blockers are cleared.