Address PR feedback: extract constants, add reset method, and real-timer test

- Extract magic numbers (500ms, 300ms) to named constants
  - shadowDomCheckTimeoutMs and shadowDomCheckDebounceMs
  - Improves maintainability and self-documentation

- Add resetObservedShadowRoots() method to DomQueryService
  - Clears WeakSet tracking when mutation observer is recreated
  - Called on navigation (handleWindowLocationMutation)
  - Prevents stale shadow root references

- Add real-timer test for debouncing behavior
  - Uses actual debounce implementation instead of mock
  - Verifies 5 rapid mutations result in only 1 page update
  - Validates debouncing works in production timing

apps/browser/src/autofill/services/abstractions/dom-query.service.ts

@@ -9,4 +9,5 @@ export interface DomQueryService {
   checkPageContainsShadowDom(): boolean;
   checkMutationsInShadowRoots(mutations: MutationRecord[]): boolean;
   checkForNewShadowRoots(): boolean;
+  resetObservedShadowRoots(): void;
 }

apps/browser/src/autofill/services/collect-autofill-content.service.spec.ts

@@ -2361,6 +2361,66 @@ describe("CollectAutofillContentService", () => {
       clearTimeout(initialTimeout);
       jest.useRealTimers();
     });
+
+    it("debounces multiple rapid shadow root mutations with real timers", (done) => {
+      jest.useRealTimers();
+
+      // Use real debounce for this test
+      const actualUtils = jest.requireActual("../utils");
+      const realDebounce = actualUtils.debounce;
+
+      const shadowHost = document.createElement("div");
+      const shadowRoot = shadowHost.attachShadow({ mode: "open" });
+      document.body.appendChild(shadowHost);
+
+      const mutationRecord: MutationRecord = {
+        type: "attributes",
+        addedNodes: document.querySelectorAll("nonexistent"),
+        attributeName: "value",
+        attributeNamespace: null,
+        nextSibling: null,
+        oldValue: null,
+        previousSibling: null,
+        removedNodes: document.querySelectorAll("nonexistent"),
+        target: shadowRoot,
+      };
+
+      collectAutofillContentService["currentLocationHref"] = window.location.href;
+
+      jest.spyOn(domQueryService, "checkMutationsInShadowRoots").mockReturnValue(true);
+
+      // Track actual calls to requirePageDetailsUpdate
+      let callCount = 0;
+      const originalRequirePageDetailsUpdate =
+        collectAutofillContentService["requirePageDetailsUpdate"];
+      collectAutofillContentService["requirePageDetailsUpdate"] = () => {
+        callCount++;
+        originalRequirePageDetailsUpdate.call(collectAutofillContentService);
+      };
+
+      // Temporarily override with real debounce
+      const originalDebounced = collectAutofillContentService["debouncedRequirePageDetailsUpdate"];
+      collectAutofillContentService["debouncedRequirePageDetailsUpdate"] = realDebounce(() => {
+        collectAutofillContentService["requirePageDetailsUpdate"]();
+      }, 300);
+
+      // Trigger 5 rapid mutations
+      for (let i = 0; i < 5; i++) {
+        collectAutofillContentService["handleMutationObserverMutation"]([mutationRecord]);
+      }
+
+      // Should only call requirePageDetailsUpdate once after debounce
+      setTimeout(() => {
+        expect(callCount).toBe(1);
+
+        // Restore original
+        collectAutofillContentService["debouncedRequirePageDetailsUpdate"] = originalDebounced;
+        collectAutofillContentService["requirePageDetailsUpdate"] =
+          originalRequirePageDetailsUpdate;
+        document.body.removeChild(shadowHost);
+        done();
+      }, 350);
+    });
   });
 
   describe("setupOverlayListenersOnMutatedElements", () => {

apps/browser/src/autofill/services/collect-autofill-content.service.ts

@@ -55,6 +55,8 @@ export class CollectAutofillContentService implements CollectAutofillContentServ
   private pendingShadowDomCheck = false;
   private ownedExperienceTagNames: string[] = [];
   private readonly updateAfterMutationTimeout = 1000;
+  private readonly shadowDomCheckTimeoutMs = 500;
+  private readonly shadowDomCheckDebounceMs = 300;
   private readonly formFieldQueryString;
   private readonly nonInputFormFieldTags = new Set(["textarea", "select"]);
   private readonly ignoredInputTypes = new Set([
@@ -1003,7 +1005,7 @@ export class CollectAutofillContentService implements CollectAutofillContentServ
       this.shadowDomCheckTimeout = setTimeout(() => {
         this.checkForNewShadowRoots();
         this.pendingShadowDomCheck = false;
-      }, 500);
+      }, this.shadowDomCheckTimeoutMs);
     }
 
     if (!this.mutationsQueue.length) {
@@ -1031,6 +1033,9 @@ export class CollectAutofillContentService implements CollectAutofillContentServ
     this._autofillFormElements.clear();
     this.autofillFieldElements.clear();
 
+    // Reset shadow root tracking on navigation
+    this.domQueryService.resetObservedShadowRoots();
+
     this.updateAutofillElementsAfterMutation();
   }
 
@@ -1074,7 +1079,7 @@ export class CollectAutofillContentService implements CollectAutofillContentServ
    */
   private debouncedRequirePageDetailsUpdate = debounce(() => {
     this.requirePageDetailsUpdate();
-  }, 300);
+  }, this.shadowDomCheckDebounceMs);
 
   /**
    * Checks for new shadow roots that aren't being observed and triggers

apps/browser/src/autofill/services/dom-query.service.ts

@@ -115,6 +115,14 @@ export class DomQueryService implements DomQueryServiceInterface {
     return false;
   };
 
+  /**
+   * Resets the observed shadow roots tracking. This should be called when the mutation
+   * observer is recreated or on significant lifecycle events (like navigation).
+   */
+  resetObservedShadowRoots = (): void => {
+    this.observedShadowRoots = new WeakSet<ShadowRoot>();
+  };
+
   /**
    * Initializes the DomQueryService, checking for the presence of shadow DOM elements on the page.
    */