Pressing space on slider resets thumb to LHS position

The issue is that a space bar press is being treated the same as a mouse click at the start of the track rather than where the thumb is. (handleTrackClick is being called with an extent containing offsetX of 0).

Start at handleTrackClick and inspect how a space-bar press supplies the extent compared with a mouse click. Reproduce the issue with a focused slider, then verify that pressing space leaves the thumb at its current position rather than moving it to the left-hand side.

Written by the indexing model from the issue text.