I think this works but deserves a comment of some sort. Ignoring horizontal scroll, we have:

```
|-----------------------* event.pageX
|-------|                 progressControl's offset parent left
        |---------------| left === event.PageX - progressControl's offset parent left
```
Then the numerator of `mouseTime` is calculated like this:
```
        |---|             progressControl.el().offsetLeft
            |-----------| mouseTime numerator
```
and all of that amounts to the difference in the current mouse position to the leftmost point of the progress control. Yes?