Every content-offset change went through onValueChanged, so a
programmatic setValue echoed the nearest tick back into the caller's
binding on open, and a width change after the first layout shifted
the reading by half the width difference.

Report only while the user is tracking, dragging or decelerating,
plus once at the end of a drag or deceleration so the snapped tick
is never lost; re-centre the last applied value when the width
changes. Adds an XCTest target with six regression cases.