To reproduce:
1. Create a new project
2. Disconnect internet
3. Open services tab
4. While it is loading, scroll to any direction(up, down, right or left).
(Mouse pointer must be on services window)

Expected vs Actual: there should not be 'white scroll'

Reproduced on: 5.5.2f1, 5.5.3p4, 5.6.1p2, 2017.1.0b7, 2017.2.0a2
Not reproducible: 5.5.0a6, 5.5.1p4

Regression introduced in 5.5.2f1

Note: this issue is reproducible only with apple's magic mouse.