Fix #1394.

1 files changed, 58 insertions, 0 deletions

diff --git a/FrontEnd/src/migrating/hooks/useReverseScrollPositionRemember.ts b/FrontEnd/src/migrating/hooks/useReverseScrollPositionRemember.ts
new file mode 100644
index 00000000..339a12b8
--- /dev/null
+++ b/FrontEnd/src/migrating/hooks/useReverseScrollPositionRemember.ts
@@ -0,0 +1,58 @@
+// Not used now!!! But preserved for future use.
+
+import { useEffect } from "react";
+
+let on = false;
+
+let rememberedReversePosition = getReverseScrollPosition();
+
+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;
+
+  window.scrollTo(0, newPosition);
+
+  document.documentElement.style.scrollBehavior = old;
+}
+
+const scrollListener = (): void => {
+  rememberedReversePosition = getReverseScrollPosition();
+};
+
+const resizeObserver = new ResizeObserver(() => {
+  scrollToReverseScrollPosition(rememberedReversePosition);
+});
+
+export default function useReverseScrollPositionRemember(): void {
+  useEffect(() => {
+    if (on) return;
+    on = true;
+    window.addEventListener("scroll", scrollListener);
+    resizeObserver.observe(document.documentElement);
+
+    return () => {
+      resizeObserver.disconnect();
+      window.removeEventListener("scroll", scrollListener);
+      on = false;
+    };
+  }, []);
+}