How to reproduce:
1. Open the project in the attached "20190410_bug_minimal.zip"
2. Make sure OpenGLCore is on top of graphics API list in PlayerSettings
3. Build standalone player
4. Run the player in full screen mode (uncheck Windowed)
5. Observe the quad
-> Quad is black (wrong)
6. Run the player again in windowed mode (check Windowed)
7. Observe the quad
-> Quad is yellow (correct)

Reproducible with: 2017.4.25f1, 2018.3.0b5, 2018.3.12f1, 2019.1.0a3, 2019.1.0a4, 2019.1.0b10, 2019.2.0a11, 2019.3.0a3

Note: The issue does not reproduce with other Graphics APIs ( Vulkan DX11 DX12 ). On 2017.4 Editor is also affected.
Note: The bug is gone if I use "yield return null" instead of "yield return new WaitForEndOfFrame()"