[Affected versions]:

• Firefox Nightly 86.0a1 (Build ID: 20201230092946)

[Affected Platforms]:

• Linux Mint 20

[Prerequisites]:

• Have Firefox Nightly open.
• You have restarted Nightly at least once before.

[Steps to reproduce]:

1. Open a new tab and navigate to a page that has login fields (e.g. “facebook.com”).
2. Attempt to login and save the credentials used.
3. Click the “Recommendation” button that has appeared in the right of the address bar.
4. Observe the borders of the doorhanger displayed.

[Expected result]:

• The doorhanger has a border that blends well with the rest of the browser and content.

[Actual result]:

• The doorhanger has a border that does not blend well with the rest of the browser and content.

[Regression]:
Last good revision: 81361995e7ed2ba370867bfbefcba44f09732d22
First bad revision: 4bc0bdcd7889aab2994b4500d41e6e35216e4848
Pushlog:
https://hg.mozilla.org/integration/autoland/pushloghtml?fromchange=81361995e7ed2ba370867bfbefcba44f09732d22&tochange=4bc0bdcd7889aab2994b4500d41e6e35216e4848

• Based on the pushlog, the issue might have been caused by Bug 1677293.

[Notes]:

• Attached is a screenshot of the issue.
• The issue is not reproducible in Beta 85 and Release 84.
• The issue is not reproducible for Windows 10 and macOS 10.15.
• Setting gfx.webrender.force-disabled to true seems to fix the issue.
• The issue was not observed on Ubuntu 20 with Compositing "Webrender" and "Basic".
• The machine the issue was observed on has Compositing "WebRender (Software)" by default.

:aosmond, could you please take a look at this?

Thanks for the report! Top left corner is round, therefore it's bug 1674473.