To reproduce:
1. Create a project, add any light
2. In light's Inspector change shadow type to soft/hard
3. Go to build settings, player settings
4. Change D3D11 fullscreen mode to exclusive mode
5. Build and run project on PC platform
6. Start the game (not windowed), defocus it (alt+tab) and focus again

Expected: game window is maximized
Actual: game window appears as black square

Reproduced in: 5.3.6p7
Not reproduced in: 5.3.5f1, 5.3.6f1, 5.3.7f1, 5.3.8f1, 5.4.5p1, 5.5.3p1, 2017.1.0b1