The distance this window has been scrolled vertically.
The Screen interface specification from W3C. scrollY from MDN.
int get scrollY => JS('bool', '("scrollY" in #)', this) ? JS('num', '#.scrollY', this).round() : document.documentElement.scrollTop;
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Licensed under the Creative Commons Attribution-ShareAlike License v4.0.
https://api.dartlang.org/stable/2.0.0/dart-html/Window/scrollY.html