How to reproduce:

1. Create a new project
2. Set Near Clipping Plane of the Main Camera to any value (e.g. 20)
3. Create a Scroll View
4. Set Canvas Plane Distance to the same value of Near Clipping Plane (e.g. 20)
5. Enter play mode and try to drag a Scrollbar

Expected behavior: You can drag a Scrollbar
Actual behavior: You can click on Scrollbar but you can't drag it

Note: You can scroll the content with mouse wheel

Reproducible: 5.3.7p2, 5.4.3p3, 5.5.0p1, 5.6.0b1