Unable to modify "Game Window Guides" while changing in the Game view.

Steps to repro:
1. Open EmptyArea scene from the attached project.
2. Select "CM vcam1" object from the hierarchy.
3. Check "Game Windows Guide" Checkbox from the inspector.
4. Try to modify Window Guides from the game view.

Observe that user unable to modify "Game Window Guides".

Reproducible on:
2018.2.0a8, 2018.2.0a7, 2018.2.0a6.

Working fine on:
2018.2.0a5, 2018.2.0a1, 2018.1.0b13.

Environment:
Occurring on Windows 10, Windows 7, Mac 10.13, Mac 10.12.