Change embedded posts to use web UI (#31766)

Co-authored-by: Claire <claire.github-309c@sitedethib.com>

app/javascript/hooks/useRenderSignal.ts

@@ -0,0 +1,32 @@
+// This hook allows a component to signal that it's done rendering in a way that
+// can be used by e.g. our embed code to determine correct iframe height
+
+let renderSignalReceived = false;
+
+type Callback = () => void;
+
+let onInitialRender: Callback;
+
+export const afterInitialRender = (callback: Callback) => {
+  if (renderSignalReceived) {
+    callback();
+  } else {
+    onInitialRender = callback;
+  }
+};
+
+export const useRenderSignal = () => {
+  return () => {
+    if (renderSignalReceived) {
+      return;
+    }
+
+    renderSignalReceived = true;
+
+    if (typeof onInitialRender !== 'undefined') {
+      window.requestAnimationFrame(() => {
+        onInitialRender();
+      });
+    }
+  };
+};