The amount that is expected to scroll horizontally, in units determined by deltaMode.
See also:
num get deltaX { if (JS('bool', '#.deltaX !== undefined', this)) { // W3C WheelEvent return this._deltaX; } throw new UnsupportedError('deltaX is not supported'); }
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https://api.dart.dev/stable/2.5.0/dart-html/WheelEvent/deltaX.html