This issue is floating and appears in a while when user works with Editor for a certain time.
Both GameView and SceneView should be visible.

To repro:
Create a simple project.
1. Add gameobject (cube, sphere, canvas etc).
2. Add some script (all projects contained Time.deltaTime()).
3. Start playmode.
4. Collapse Editor (?).

Actual result: If you expand Editor, Gameview will blackout (like no Display is selected).
Expected result: Gameview is not hidden.

Workaround:
Launch the editor with -force-gfx-direct flag.

Regression since 5.3.
Is present in 5.4 and trunk.
Tested on 5.4.0b24-b25-0f1-0f3, 5.5.0a3-5.5.0a5.

See forum thread for additional info: http://forum.unity3d.com/threads/blank-game-view.421817/