We were always setting `scrubbing(true)` on mouse down. This means, that
we'd use `fastSeek` even when seeking while clicking, rather than only
when scrubbing.
The main fix involves knowing in `handleMouseMove` whether we were
called directly as a `mousemove` handler or whether it was called from
`handleMouseDown`. This means that when `handleMouseMove` is called via
`handleMouseDown` we can skip setting `scrubbing(true)` and only do it
when we are scrubbing directly.
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