If a panel is being displayed over the scene view (Particle Effects, Tools etc), it seems as if their box is stretched across the entire width of the Scene View. This blocks user input in any area of the scene within the height of the panel.

1. Open Unity
2. Open the attached project
3. Open SampleScene.scene
4. Select the Particle System in the hiearchy so the Particle Effect Panel is displayed in the scene view
5. In Scene View, try any user input in the area to the left of the Particle Effect Panel e.g. holding the right mouse button to look around the scene

Expected Result:
User input is accepted.

Actual Result:
User Input is blocked.

Tested on Windows.

Occurs on 2019.3.0a1, 2019.2.0a12