Reproduction steps:
1. Open the attached project;
2. Open the scene "blackflicker";
3. Resize the Game window;

Actual result: the view in the Game window flickers (OnPostRender_actualResult.gif).
Expected result: the view in the Game window should not flicker (OnPostRender_expectedResult.gif).

Note: When the camera is selected and CTRL + SHIFT + F is pressed, the Game window becomes black. If you click on the black screen, the view shows up. The camera has a post FX filter that sets a rendertexture in OnPreRender and blits it in OnPostRender.
In the 5.6.0a1 - 5.6.0b4 versions when the project is opened, the Game window always is black.

Reproduced with: 5.6.0a1, 5.6.0b8
Not reproduced with: 5.4.4f1, 5.5.1p4
Regression introduced in 5.6.0a1