- Open Player Settings and set Default cursor in Cross-Platform Settings
- Open e.g. Build Settings window and drag it over Game view or Scene View
- Click "Play"
- See that OS cursor is overridden with custom one and when mouse is being moved cursors "compete" with each other

Another repro:
1) Open gradient editor (from a particle system)
2) Ensure its over Scene or Game View
3) Place cursor over alpha label and see its flickering when moving
Note that you do not need to be Play mode