"Default Screen Width and Default Screen Height are used by the Web Player when entering fullscreen mode through the context menu in the Web Player at runtime." (http://unity3d.com/support/documentation/Components/class-PlayerSettings.html).

When a browser game enters fullscreen the resolution on the primary monitor changes to the one set in Default Screen Width/Height, which is usually a low resolution. The amount of flickering you'll get during that context switch seems to depend on the gfx card and the monitors used.

The default behaviour (using Default Screen Width/Height as fullscreen resolution) is just weird. We cannot change it now, since that might possibly break existing content. We might add a helper function that would setup Default Screen Width/Height to the actual screen resolution, but it would probably need to take care of the aspect ratio and such as well.

Extracted from case 388923.