Bug 1789235: Fire a scrolling start event for anchor jump if focus in doc, r=Jamie

This revision modifies NotifyOfAnchorJump in order to ensure that we fire a
scrolling start event for AT clients. Without this, clients might miss anchor
jump updates, since anchor jumps can arrive after getting focus. This revision
also adds a test to verify that the scrolling start event is now being sent.

Differential Revision: https://phabricator.services.mozilla.com/D177419

accessible/base/nsAccessibilityService.cpp

@@ -486,8 +486,24 @@ nsAccessibilityService::Observe(nsISupports* aSubject, const char* aTopic,
 void nsAccessibilityService::NotifyOfAnchorJumpTo(nsIContent* aTargetNode) {
   Document* documentNode = aTargetNode->GetUncomposedDoc();
   if (documentNode) {
+    // If the document has focus when we get this notification, ensure that
+    // we fire a start scrolling event.
     DocAccessible* document = GetDocAccessible(documentNode);
-    if (document) document->SetAnchorJump(aTargetNode);
+    const Accessible* focusedAcc = FocusedAccessible();
+    if (focusedAcc && focusedAcc == document) {
+      LocalAccessible* targetAcc = document->GetAccessible(aTargetNode);
+      if (targetAcc) {
+        nsEventShell::FireEvent(nsIAccessibleEvent::EVENT_SCROLLING_START,
+                                targetAcc);
+        document->SetAnchorJump(nullptr);
+      } else {
+        // We can't find the target accessible in the document yet. Set the
+        // anchor jump so that we can fire the scrolling start event later.
+        document->SetAnchorJump(aTargetNode);
+      }
+    } else if (document) {
+      document->SetAnchorJump(aTargetNode);
+    }
   }
 }
 

accessible/tests/browser/events/browser_test_scrolling.js

@@ -111,3 +111,20 @@ c</textarea>
     await onScrolling;
   }
 );
+
+// Verify that the scrolling start event is fired for an anchor change.
+addAccessibleTask(
+  `
+    <p>a</p>
+    <p>b</p>
+    <p id="c">c</p>
+  `,
+  async function (browser, accDoc) {
+    let onScrollingStart = waitForEvent(EVENT_SCROLLING_START, "c");
+    await SpecialPowers.spawn(browser, [], () => {
+      content.location.hash = "#c";
+    });
+    await onScrollingStart;
+  },
+  { chrome: true, topLevel: true }
+);