Reproduction steps:
1. Open the attached Project “UI_Flicker”
2. Open the “Assets/Scene/E3_09_LnL.unity”
3. Add a material “Assets/MAT_Test.mat” onto the “Sphere” GameObject
4. Position Scene and Game views so they would be both visible
5. Move your mouse to the Game view and then to the Scene view a couple of times

Expected result: Editor UI is stable
Actual result: Scene view toolbar appearance changes whenever the mouse is in the Game view

Reproducible with: 2021.2.0a17, 2021.3.21f1, 2022.2.12f1, 2023.1.0b9, 2023.2.0a8
Not reproducible with: 2020.3.47f1, 2021.2.0a16

Reproducible on: Windows 10

Note: This bug affects Package Manager as well