The distance this window has been scrolled horizontally.
The Screen interface specification from W3C. scrollX from MDN.
int get scrollX => JS('bool', '("scrollX" in #)', this) ? JS('num', '#.scrollX', this).round() : document.documentElement.scrollLeft;
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https://api.dartlang.org/stable/2.0.0/dart-html/Window/scrollX.html