🐛 [RUMF-469] fix angular compatibility

packages/rum/src/domMutationCollection.ts

@@ -1,13 +1,46 @@
 import { monitor } from '@datadog/browser-core'
 import { LifeCycle, LifeCycleEventType } from './lifeCycle'
 
+type MutationObserverConstructor = new (callback: MutationCallback) => MutationObserver
+
 interface BrowserWindow extends Window {
-  MutationObserver?: MutationObserver
+  MutationObserver?: MutationObserverConstructor
+  Zone?: {
+    __symbol__(name: string): string
+  }
+}
+
+function getMutationObserverConstructor(): MutationObserverConstructor | undefined {
+  let constructor: MutationObserverConstructor | undefined
+  const browserWindow: BrowserWindow = window
+
+  // Angular uses Zone.js to provide a context persisting accross async tasks.  Zone.js replaces the
+  // global MutationObserver constructor with a patched version to support the context propagation.
+  // There is an ongoing issue[1][2] with this setup when using a MutationObserver within a Angular
+  // component: on some occasions, the callback is being called in an infinite loop, causing the
+  // page to freeze (even if the callback is completely empty).
+  //
+  // To work around this issue, we are using the Zone __symbol__ API to get the original, unpatched
+  // MutationObserver constructor.
+  //
+  // [1] https://github.com/angular/angular/issues/26948
+  // [2] https://github.com/angular/angular/issues/31712
+  if (browserWindow.Zone) {
+    const symbol = browserWindow.Zone.__symbol__('MutationObserver')
+    constructor = browserWindow[symbol as any] as any
+  }
+
+  if (!constructor) {
+    constructor = browserWindow.MutationObserver
+  }
+
+  return constructor
 }
 
 export function startDOMMutationCollection(lifeCycle: LifeCycle) {
   let observer: MutationObserver | undefined
-  if ((window as BrowserWindow).MutationObserver) {
+  const MutationObserver = getMutationObserverConstructor()
+  if (MutationObserver) {
     observer = new MutationObserver(
       monitor(() => {
         lifeCycle.notify(LifeCycleEventType.DOM_MUTATED)