Fix render loop in <Modal>, clean up ref merger code

ts/util/refMerger.ts

@@ -0,0 +1,35 @@
+// Copyright 2021 Signal Messenger, LLC
+// SPDX-License-Identifier: AGPL-3.0-only
+
+import type { MutableRefObject, Ref } from 'react';
+import memoizee from 'memoizee';
+
+/**
+ * Merges multiple refs.
+ *
+ * Returns a new function each time, which may cause unnecessary re-renders. Try
+ * `createRefMerger` if you want to cache the function.
+ */
+export function refMerger<T>(
+  ...refs: Array<Ref<unknown>>
+): (topLevelRef: T) => void {
+  return (el: T) => {
+    refs.forEach(ref => {
+      // This is a simplified version of [what React does][0] to set a ref.
+      // [0]: https://github.com/facebook/react/blob/29b7b775f2ecf878eaf605be959d959030598b07/packages/react-reconciler/src/ReactFiberCommitWork.js#L661-L677
+      if (typeof ref === 'function') {
+        ref(el);
+      } else if (ref) {
+        // I believe the types for `ref` are wrong in this case, as `ref.current` should
+        //   not be `readonly`. That's why we do this cast. See [the React source][1].
+        // [1]: https://github.com/facebook/react/blob/29b7b775f2ecf878eaf605be959d959030598b07/packages/shared/ReactTypes.js#L78-L80
+        // eslint-disable-next-line no-param-reassign
+        (ref as MutableRefObject<T>).current = el;
+      }
+    });
+  };
+}
+
+export function createRefMerger(): typeof refMerger {
+  return memoizee(refMerger, { length: false, max: 1 });
+}