The amount that is expected to scroll horizontally, in units determined by deltaMode.
See also:
WheelEvent.deltaX from the W3C.
Source
@DomName('WheelEvent.deltaX') num get deltaX { if (JS('bool', '#.deltaX !== undefined', this)) { // W3C WheelEvent return this._deltaX; } throw new UnsupportedError('deltaX is not supported'); }