1505373 - [Mac 10.14] Pixels light up between the menu bar and tab bar

Status: RESOLVED DUPLICATE of bug 1494058

(Core :: Widget: Cocoa, defect)

Description

[laszlo.bialis]

[Note]:
- I've only managed to reproduce the issue on a MacBook with the following config:
macOS Mojave v.10.14
MacBook Pro (13-inch, 2017, Two Thunderbolt 3 ports)
Processor 2.3GHz Intel Core i5
Memory 8GB 2133MHz LPDDR3
Graphics Intel Iris Plus Graphics 640 1536 MB

[Affected versions]:
Nightly 65.0a1 20181107100135, Beta 64.0b7 20181105164654, Release 63.0.1 20181030165643

[Affected platforms]:
MacOS 10.14

[Steps to reproduce]:
1. Set MacOS 10.14 to dark theme (system preferences/general/appearance:dark)
2. Open FF using default theme, make sure that the browser window fills the screen, but it's not in full screen mode (the OS menu bar is visible) 
3. Open several tabs
4. Open hotnews.ro in a new tab 
5. Open youtube.com in a new tab and play a video and wait a few minutes
6. Examine the line between the menu bar and the top of the tab

[Expected result]:
- The line between the menu bar and the top of the tab should be dark, the only different colored line should be the selected tab's blue line.

[Actual result]:
- Some pixels light up above an inactive tab, between the menu bar line (usually orange or red colored) 
- If it's not reproducible at the firs try just repeat the steps, usually the pixels appear if no other activity is done in the same time (thus I couldn't take a screenshot)
- I've uploaded a video here: https://streamable.com/8snl4 the issue is not very visible (follow the mouse pointer), the original video is 53MB but I can send it if needed