Steps to reproduce:
1. Open attached project
2. Go to Window -> Test Window
3. Start play mode
4. Select "Main Camera" gameObject in hierarchy ("TestWindow" now should start rendering camera's view)
5. Scale "Test Window" down a few times (see in the attached GIF)

Actual result: The part(s) in Game view are not cleared up where camera does not render.
Expected result: Not rendered parts should be cleared up.

Note: Game view clears up when is rescaled(see in the attached GIF)

Reproduce on: 5.5.0b4, 5.4.1f1

Does not reproduce on: 5.3.6p5