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node_modules/popper.js/src/utils/getScrollParent.js

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+import getStyleComputedProperty from './getStyleComputedProperty';
+import getParentNode from './getParentNode';
+
+/**
+ * Returns the scrolling parent of the given element
+ * @method
+ * @memberof Popper.Utils
+ * @argument {Element} element
+ * @returns {Element} scroll parent
+ */
+export default function getScrollParent(element) {
+  // Return body, `getScroll` will take care to get the correct `scrollTop` from it
+  if (!element) {
+    return document.body
+  }
+
+  switch (element.nodeName) {
+    case 'HTML':
+    case 'BODY':
+      return element.ownerDocument.body
+    case '#document':
+      return element.body
+  }
+
+  // Firefox want us to check `-x` and `-y` variations as well
+  const { overflow, overflowX, overflowY } = getStyleComputedProperty(element);
+  if (/(auto|scroll|overlay)/.test(overflow + overflowY + overflowX)) {
+    return element;
+  }
+
+  return getScrollParent(getParentNode(element));
+}