Steps to reproduce:
1. Open attached Unity Project "Test2d"
2. Load Scene "Test2D"
3. Zoom in until the red Icon in the viewport disappears
4. Turn the slider under Gizmos to the left until the Icon reappears
5. Zoom out

Note: This can be repeated many times by zooming in and out - each one would require a manual readjustment of the Scale slider.

Expected result: 3D Gizmos are visible under any zoom/scale level
Actual result: 3D Gizmos disappear (see attached gif "3dgizmos.gif" for a preview)

Reproducible with: 2017.1.0b1, 2017.1.0b5, 2017.1.0b9, 2017.1.0b10, 2017.2.0a3
Not reproducible with: 5.6.0f3, 5.6.1p4, 5.6.2f1, 2017.1.0a2, 2017.1.0a6
Regression introduced in: 2017.1.0b1