To reproduce:

1. Download attached project "ViewportTesting_NoVR.zip" and open in Unity
2. Open "test" scene
3. Enter to Play mode
4. Select "Main Camera" game object
5. Double click on "Target Texture" field

Expected result: In Inspector window you will see that only the left half of the texture is be rendered
Actual result: In Inspector window you will see that the full width of the texture is be rendered and a Sphere game object is squeezed

Reproduced on Unity 5.6.0p4, 5.6.1p1 and 2017.2.0a1
Not reproduced on Unity 5.5.3p4, 5.6.0p3, 2017.1.0a2 and 2017.1.0b7
Regression on Unity 5.6.0p4