How to reproduce:
1. Open the "Repro.zip" project
2. Open the "SampleScene" scene
3. Window > Analysis > Frame Debugger
4. Enable the Frame Debugger
5. In the Hierarchy window, start moving mouse sideways so it gets bigger (macOS feature)
6. Hover the mouse over Game Objects in the scene
7. Hover the mouse over Editor Tools UI elements

Actual result: Game and Frame Debugger windows start flickering. (Gif attached)
Expected result: The windows are not flickering.

Reproducible with: 2020.1.0a6, 2019.3.0b5, 2019.3.0a3.
Regression introduced in: 2019.3.0a3.
Not reproducible with: 2019.3.0a2, 2019.3.0a1, 2019.2.7f2, 2018.4.10f1, 2017.4.32f1.

Notes:
- Hovering over Editor Tools UI elements seems to have higher chance to trigger the flicker.
- Seems like the flicker starts more often when/after the cursor gets bigger after shaking it.
- Not reproducible on Windows