Default behavior for LMB+drag in any timeline panel (timeline, dope sheet, action editor etc.) in Blender 2.8 is to create a selection box instead of scrubbing through animation. I have the B key to enter box select if I want to select things - I don't need this new method of selecting things.
I've managed to disable the selection box, but the scrub still doesn't work - now if I hold LMB and drag on the timeline it just does nothing. And I don't want to have to click on the frame number panel (which does what I want it to when I click and drag on that - see picture for clearer explanation).