To reproduce:

1. Download attached project "Unity_563p1_ScrollbarBug.zip" and open in Unity
2. Open "scrollbar_bug" scene
3. Enter Play mode
4. In Game window move Scrollbar to bottom
5. Resize Window
Note: Observe that Scrollbar disappears because it is not necessary anymore

Expected Result: Scrollcontent does not get cut off
Actual Result: Scrollcontent gets cut off

Does not reproduce on Unity 5.6.0p2
Reproduced on Unity 5.6.0p3, 5.6.3p2, 2017.1.1f1, 2017.2.0b10 and 2017.3.0a7
Regression introduced on Unity 5.6.0p3

A fix is being reviewed at: https://ono.unity3d.com/unity/unity/pull-request/52615/_/ui/bugfix/case-948085-scrollcontent-cut.