To reproduce:

1. Import the package attached by the tester (ScrollBarBug.zip)
2. Open the "BugScene" scene
3. Enter Play mode
4. Expand Canvas in the Hierarchy and select Bug Report Prefab
5. In the Scene view, grab the right edge of the object and quickly expand and contract it horizontally
7. Observe how scrollbar appears out of position for a couple of frames

Expected: scrollbar doesn't appear out of position for a few seconds

Reproduced in 2017.1.0p5, 2017.1.2f1
Did not reproduce in 5.5.5f1, 5.6.4f1, 2017.2.0b6, 2017.2.0f3, 2017.3.0b4
Regression introduced in 2017.1.0b9

Not reproducible in 2017.2.0b6