fix(core): handle cancelled traversals in fake navigation

Update fake navigation implementation to correctly handle traversals that are cancelled (e.g. by a precommit handler).
Ensure prospective index is calculated correctly so that subsequent traversals target the correct entry.
Add regression test for cancelled traversals.

Author: atscott

packages/core/primitives/dom-navigation/testing/fake_navigation.ts

@@ -61,7 +61,7 @@ export class FakeNavigation implements Navigation {
    * A prospective current active entry index, which includes unresolved
    * traversals. Used by `go` to determine where navigations are intended to go.
    */
-  private prospectiveEntryIndex = 0;
+  private propsectiveTraversalDestinations: number[] = [];
 
   /**
    * A test-only option to make traversals synchronous, rather than emulate
@@ -300,7 +300,7 @@ export class FakeNavigation implements Navigation {
       index: entry.index,
       sameDocument: entry.sameDocument,
     });
-    this.prospectiveEntryIndex = entry.index;
+    this.propsectiveTraversalDestinations.push(entry.index);
     const result = new InternalNavigationResult(this);
     this.traversalQueue.set(entry.key, result);
     this.runTraversal(() => {
@@ -366,11 +366,13 @@ export class FakeNavigation implements Navigation {
    * `back(); forward()` chains it collapses certain traversals.
    */
   go(direction: number): void {
-    const targetIndex = this.prospectiveEntryIndex + direction;
+    const targetIndex =
+      (this.propsectiveTraversalDestinations[this.propsectiveTraversalDestinations.length - 1] ??
+        this.currentEntryIndex) + direction;
     if (targetIndex >= this.entriesArr.length || targetIndex < 0) {
       return;
     }
-    this.prospectiveEntryIndex = targetIndex;
+    this.propsectiveTraversalDestinations.push(targetIndex);
     this.runTraversal(() => {
       // Check again that destination is in the entries array.
       if (targetIndex >= this.entriesArr.length || targetIndex < 0) {
@@ -407,6 +409,7 @@ export class FakeNavigation implements Navigation {
   private runTraversal(traversal: () => void) {
     if (this.synchronousTraversals) {
       traversal();
+      this.propsectiveTraversalDestinations.shift();
       return;
     }
 
@@ -418,6 +421,7 @@ export class FakeNavigation implements Navigation {
         setTimeout(() => {
           resolve();
           traversal();
+          this.propsectiveTraversalDestinations.shift();
         });
       });
     });
@@ -558,7 +562,7 @@ export class FakeNavigation implements Navigation {
       }
     } else if (navigationType === 'push') {
       this.currentEntryIndex++;
-      this.prospectiveEntryIndex = this.currentEntryIndex; // prospectiveEntryIndex isn't in the spec but is an implementation detail
+      this.propsectiveTraversalDestinations = []; // prospectiveEntryIndex isn't in the spec but is an implementation detail
       disposedNHEs.push(...this.entriesArr.splice(this.currentEntryIndex));
     } else if (navigationType === 'replace') {
       disposedNHEs.push(oldCurrentNHE);

packages/core/primitives/dom-navigation/testing/test/fake_platform_navigation.spec.ts

@@ -1464,6 +1464,37 @@ describe('navigation', () => {
       expect(secondTraverseFinishedEntry).toBe(secondPageEntry);
       expect(locals.navigation.currentEntry).toBe(secondPageEntry);
     });
+
+    it('subsequent traversal works after cancelled traversal', async () => {
+      const [firstPageEntry, secondPageEntry, thirdPageEntry] = await setUpEntries();
+      // Go back to the first page so we have room to traverse forward
+      await locals.navigation.traverseTo(firstPageEntry.key).finished;
+      locals.navigateEvents.length = 0;
+      locals.navigationCurrentEntryChangeEvents.length = 0;
+      locals.popStateEvents.length = 0;
+
+      // Try to traverse to the second page but cancel it
+      locals.pendingInterceptOptions.push({
+        precommitHandler: () => Promise.reject(new Error('cancelled')),
+      });
+      const {committed, finished} = locals.navigation.traverseTo(secondPageEntry.key);
+      await expectAsync(committed).toBeRejectedWithError(Error, /cancelled/);
+      await expectAsync(finished).toBeRejectedWithError(Error, /cancelled/);
+      expect(locals.navigation.currentEntry).toBe(firstPageEntry);
+
+      // Verify that we can still traverse to the third page correctly
+      // Using go(2) ensures that the prospective index was reset correctly
+      // (index 0 -> index 2 relative to current)
+      locals.navigation.go(2);
+      // We need to wait for the traversal to happen. go() is void, but we can check the next event.
+      const navigateEvent = await locals.nextNavigateEvent();
+      expect(navigateEvent.destination.key).toBe(thirdPageEntry.key);
+      // Determine if we need to await committed/finished for go() or if nextNavigateEvent is enough.
+      // go() uses internal mechanism, but eventually updates currentEntry.
+      // We can wait for the loop to settle.
+      await new Promise((resolve) => setTimeout(resolve, 0));
+      expect(locals.navigation.currentEntry.key).toBe(thirdPageEntry.key);
+    });
   });
 
   describe('history API', () => {