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diff --git a/popperjs/package/lib/dom-utils/getWindowScrollBarX.js.flow b/popperjs/package/lib/dom-utils/getWindowScrollBarX.js.flow
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+++ b/popperjs/package/lib/dom-utils/getWindowScrollBarX.js.flow
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+// @flow
+import getBoundingClientRect from './getBoundingClientRect';
+import getDocumentElement from './getDocumentElement';
+import getWindowScroll from './getWindowScroll';
+
+export default function getWindowScrollBarX(element: Element): number {
+  // If <html> has a CSS width greater than the viewport, then this will be
+  // incorrect for RTL.
+  // Popper 1 is broken in this case and never had a bug report so let's assume
+  // it's not an issue. I don't think anyone ever specifies width on <html>
+  // anyway.
+  // Browsers where the left scrollbar doesn't cause an issue report `0` for
+  // this (e.g. Edge 2019, IE11, Safari)
+  return (
+    getBoundingClientRect(getDocumentElement(element)).left +
+    getWindowScroll(element).scrollLeft
+  );
+}