Fix vertical scrolling issues.

layouts/scripts/frame/editor.js

@@ -0,0 +1,17 @@
+export function setupFrameResizeListener() {
+  const previewFrame = getPreviewFrame();
+
+  window.addEventListener('message', function (e) {
+    const RESIZE_CODE = 'resize:';
+    if (typeof e.data !== 'string' || !e.data.startsWith(RESIZE_CODE)) {
+      return;
+    }
+
+    const data = JSON.parse(e.data.substring(RESIZE_CODE.length))
+    previewFrame.style.height = data.height + 'px'
+  });
+}
+
+export function getPreviewFrame() {
+  return document.getElementById('preview_frame');
+}

layouts/scripts/frame/frame.js

@@ -0,0 +1,32 @@
+'use strict';
+
+// Scrollbars / Frame resizes notifications.
+(function () {
+  let height;
+  const heightLimit = window.outerHeight * 2;
+
+  handleHeightChange(); // Initial frame's height setup.
+  setupResizeListener(); // Listen to frame's height changes.
+
+  ///
+
+  function setupResizeListener() {
+    const resizeObserver = new ResizeObserver(handleHeightChange);
+    resizeObserver.observe(document.body);
+  }
+
+  function handleHeightChange(entries) {
+    const updatedHeight = getCurrentHeight();
+    if (height === updatedHeight || updatedHeight > heightLimit) {
+      return;
+    }
+
+    const RESIZE_CODE = 'resize:';
+    height = updatedHeight;
+    window.parent.postMessage(RESIZE_CODE + JSON.stringify({height}), '*');
+  }
+
+  function getCurrentHeight() {
+    return document.querySelector('body > main').scrollHeight;
+  }
+})();