first commit

common/resources/client/utils/dom/observe-size.ts

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+import {RefObject} from 'react';
+
+type Callback = (e: {width: number; height: number}) => void;
+
+export function observeSize(
+  ref: RefObject<HTMLElement>,
+  callback: Callback
+): () => void {
+  const observer = new ResizeObserver(entries => {
+    const rect = entries[0].contentRect;
+    callback({width: rect.width, height: rect.height});
+  });
+  if (ref.current) {
+    observer.observe(ref.current);
+  }
+  return () => {
+    if (ref.current) {
+      observer.unobserve(ref.current);
+    }
+  };
+}