How to reproduce:
1. Open project "2018303.zip"
It's not that simple to explain what combination of RMB and arrow keys to press, so let's use additional software.
2. Install attached MouseKeyboardRecorder.exe
3. Open Mouse and Keyboard Recorder, File > Open and select attached file "ReproClicks".
4. While in Editor, press "Repeat" in Mouse And Keyboard Recorder and it should reproduce the issue.
5. When it does, press Mouse 2 in the Scene View.

Expected result: scene view position does not change when only pressing right mouse button.
Actual result: scene view position does change when only pressing right mouse button.

Reproducible with - 2018.3.0a4, 2018.2.0f1, 2018.2.0b2
Not reproducible with - 2018.2.0b1, 2018.1.7f1, 2017.4.6f1, 2017.2.3p2, 2017.1.4p2