Add getClientRects to fabric fragment instance (facebook#34545)

Stacked on facebook#34544 

We only have getBoundingClientRect available from RN currently. This
should work as a substitute for this case because the equivalent of
multi-rect elements in RN is a nested Text component. We only include
the rects of top-level host components here so we can assume that
calling getBoundingClientRect on each child is the same result.

Tested in react-native with Fantom.

Author: jackpope

packages/react-native-renderer/src/ReactFiberConfigFabric.js

@@ -648,6 +648,7 @@ export type FragmentInstanceType = {
   getRootNode(getRootNodeOptions?: {
     composed: boolean,
   }): Node | FragmentInstanceType,
+  getClientRects: () => Array<DOMRect>,
 };
 
 function FragmentInstance(this: FragmentInstanceType, fragmentFiber: Fiber) {
@@ -772,6 +773,27 @@ FragmentInstance.prototype.getRootNode = function (
   return rootNode;
 };
 
+// $FlowFixMe[prop-missing]
+FragmentInstance.prototype.getClientRects = function (
+  this: FragmentInstanceType,
+): Array<DOMRect> {
+  const rects: Array<DOMRect> = [];
+  traverseFragmentInstance(this._fragmentFiber, collectClientRects, rects);
+  return rects;
+};
+function collectClientRects(child: Fiber, rects: Array<DOMRect>): boolean {
+  const instance = getPublicInstanceFromHostFiber(child);
+
+  // getBoundingClientRect is available on Fabric instances while getClientRects is not.
+  // This should work as a substitute in this case because the only equivalent of a multi-rect
+  // element in RN would be a nested Text component.
+  // Since we only use top-level nodes here, we can assume that getBoundingClientRect is sufficient.
+  // $FlowFixMe[method-unbinding]
+  // $FlowFixMe[incompatible-use] Fabric PublicInstance is opaque
+  rects.push(instance.getBoundingClientRect());
+  return false;
+}
+
 export function createFragmentInstance(
   fragmentFiber: Fiber,
 ): FragmentInstanceType {