Open up about tracing, load a lof (Like the one in  issue 592950 ).  Then choose the pointer, and select an even, or select a bunch of events.  When you do, the scroll position of main tracing window moves all the way to the top, so you lose your place.

I can repro this on Windows beta (50.0.2661.49) and Canary (51.0.2694.1).