Steps to reproduce:
1. Pause the debugger
2. Hit "step over" several times quickly.
It seems that when later lines are highlighted, the animation/transition is restarted for prior highlights. So if you step through code quickly, many lines will flash at once.
*** Bug 141415 has been marked as a duplicate of this bug. ***
See bug 141415 for an animated GIF.
This affects jumping to records in the timelines too.
Highlight animation has been removed.