If you use the playhead to scrub (saying playing through a lip sync sequence) you can only scrub within the currently displayed timeline view. Once you get to the end, you have to stop, horizontally scroll the timeline and then continue scrubbing again. This is the same in reverse.

Zooming the timeline isn't really a solution, as it messes up how you want to view it while scrubbing.

My suggestion is that if you click and drag the playhead and get to either end, the timeline should scroll along with it.