Add missing script file.

scripts/gen_site.sh

@@ -41,6 +41,7 @@ babel --source-maps --minified --no-comments \
   tmp/js/codeBlocks.js \
   tmp/js/links.js \
   tmp/js/readTracking.js \
+  tmp/js/resizeObserver.js \
   tmp/js/scroll.js \
   tmp/js/overlays.js \
   tmp/js/lang.js \

src/ts/resizeObserver.js

@@ -0,0 +1,32 @@
+// Copyright Istio Authors
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//     http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+"use strict";
+
+function observeResize(el, callback) {
+    if (window.ResizeObserver) {
+       const ro = new ResizeObserver(entries => {
+            for (let entry of entries) {
+                callback(entry.target);
+            }
+       });
+
+       ro.observe(el);
+    } else {
+        // simple fallback...
+        listen(window, "resize", () => {
+            callback(el);
+        });
+    }
+}