How to reproduce:
1. Download attached project and open "Test" Scene
2. Make sure settings in Player settings are: D3D Fullscreen Mode: Fullscreen Window; Visible in Background: Off
3. Press build and run
4. In game notice actual resolution. Press "Set 1024x768" and then alt + tab. Then go back to game and notice, that resolution is not 1024x768 anymore

Expected result: Resolution should not change after losing focus
Actual result: When launching the game in fullscreen mode or changing settings during play, the resolution is set correctly. However, once the window loses focus (eg. alt + tab), the window will restore itself with the native resolution of the attached display

Reproduced with: 2017.1.1p1, 2017.2.0b11, 2017.3.0a7
Not reproduced with: 5.6.3p3

Regression since: 2017.1.0a6 (can't test with a1-a5, because game not launched properly)