Affected versions

• Firefox 68.0a1 (20190514214217)
• Firefox 67.0b19 (20190509185224)
• Firefox 66.0.5 (20190507012018)

Affected platforms

• Ubuntu 18.04

Steps to reproduce

1. Open Firefox and open a new window.
2. Open https://www.facebook.com/ on both windows.
3. Drag each window to the corner of the screen where you want it.

Expected result

• The browser windows has a border and a shadow to differentiate one from another.

Actual result

• There is no clear delimitation between browser windows.

Regression Range

• Last good revision: 826b59e57fe4274954088e7a9ed9bab092203e1c (2019-02-22)
• First bad revision: 492e4409f468f4841def795627356148d0bb7347 (2019-02-23)
• Pushlog:
  https://hg.mozilla.org/mozilla-central/pushloghtml?fromchange=826b59e57fe4274954088e7a9ed9bab092203e1c&tochange=492e4409f468f4841def795627356148d0bb7347
• Potential regressor: bug 1529713.

Additional Notes

• This issue only occurs when the browser windows are dragged in half screen.
• The issue is not reproducible when the browser windows are dragged near each other without activating half screen feature (the shadow delimitation is present).
• Attached a screen recording with the issue.

Bulk change of P3 carryover bugs to wontfix for 68.