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1 files changed, 77 insertions, 0 deletions

diff --git a/FrontEnd/src/utilities/useReverseScrollPositionRemember.ts b/FrontEnd/src/utilities/useReverseScrollPositionRemember.ts
new file mode 100644
index 00000000..a5812808
--- /dev/null
+++ b/FrontEnd/src/utilities/useReverseScrollPositionRemember.ts
@@ -0,0 +1,77 @@
+import React from "react";
+
+let on = false;
+
+let reverseScrollPosition = getReverseScrollPosition();
+let reverseScrollToPosition: number | null = null;
+let lastScrollPosition = window.scrollY;
+
+export function getReverseScrollPosition(): number {
+  if (document.documentElement.scrollHeight <= window.innerHeight) {
+    return 0;
+  } else {
+    return (
+      document.documentElement.scrollHeight -
+      document.documentElement.scrollTop -
+      window.innerHeight
+    );
+  }
+}
+
+export function scrollToReverseScrollPosition(reversePosition: number): void {
+  if (document.documentElement.scrollHeight <= window.innerHeight) return;
+
+  const old = document.documentElement.style.scrollBehavior;
+  document.documentElement.style.scrollBehavior = "auto";
+
+  const newPosition =
+    document.documentElement.scrollHeight -
+    window.innerHeight -
+    reversePosition;
+
+  reverseScrollToPosition = newPosition;
+
+  window.scrollTo(0, newPosition);
+
+  document.documentElement.style.scrollBehavior = old;
+}
+
+const scrollListener = (): void => {
+  if (
+    reverseScrollToPosition != null &&
+    Math.abs(window.scrollY - reverseScrollToPosition) > 50
+  ) {
+    scrollToReverseScrollPosition(reverseScrollPosition);
+    return;
+  }
+  if (
+    reverseScrollToPosition == null &&
+    Math.abs(window.scrollY - lastScrollPosition) > 1000
+  ) {
+    scrollToReverseScrollPosition(reverseScrollPosition);
+    return;
+  }
+
+  reverseScrollToPosition = null;
+  lastScrollPosition = window.scrollY;
+  reverseScrollPosition = getReverseScrollPosition();
+};
+
+const resizeObserver = new ResizeObserver(() => {
+  scrollToReverseScrollPosition(reverseScrollPosition);
+});
+
+export default function useReverseScrollPositionRemember(): void {
+  React.useEffect(() => {
+    if (on) return;
+    on = true;
+    window.addEventListener("scroll", scrollListener);
+    resizeObserver.observe(document.documentElement);
+
+    return () => {
+      window.removeEventListener("scroll", scrollListener);
+      resizeObserver.disconnect();
+      on = false;
+    };
+  }, []);
+}