Bug 1691813 - Use TreeWalker to find accessible descendants. r=Jamie

This speeds up reflows on pages like view source that have a single
giant container. But this maybe slows down other cases? I'm not 100%
sure.

Differential Revision: https://phabricator.services.mozilla.com/D117948

accessible/generic/DocAccessible.cpp

@@ -1377,20 +1377,10 @@ LocalAccessible* DocAccessible::GetAccessibleOrDescendant(
     return const_cast<DocAccessible*>(this);
   }
 
-  acc = GetContainerAccessible(aNode);
-  if (acc) {
-    // We access the `mChildren` array directly so that we don't access
-    // lazily created children in places like `XULTreeAccessible` and
-    // `XULTreeGridAccessible`.
-    uint32_t childCnt = acc->mChildren.Length();
-    for (uint32_t idx = 0; idx < childCnt; idx++) {
-      LocalAccessible* child = acc->mChildren.ElementAt(idx);
-      for (nsIContent* elm = child->GetContent();
-           elm && elm != acc->GetContent();
-           elm = elm->GetFlattenedTreeParent()) {
-        if (elm == aNode) return child;
-      }
-    }
+  if (acc = GetContainerAccessible(aNode)) {
+    TreeWalker walker(acc, aNode->AsContent(),
+                      TreeWalker::eWalkCache | TreeWalker::eScoped);
+    return walker.Next();
   }
 
   return nullptr;

accessible/interfaces/nsIAccessibilityService.idl

@@ -33,6 +33,8 @@ interface nsIAccessibilityService : nsISupports
    */
   nsIAccessible getAccessibleFor(in Node aNode);
 
+  nsIAccessible getAccessibleDescendantFor(in Node aNode);
+
    /**
     * Returns accessible role as a string.
     *

accessible/tests/mochitest/tree/test_divs.html

@@ -16,6 +16,10 @@
         src="../attributes.js"></script>
 
 <script type="application/javascript">
+function getAccessibleDescendantFor(selector) {
+  return gAccService.getAccessibleDescendantFor(document.querySelector(selector));
+}
+
 function doTest() {
   // All below test cases are wrapped in a div which always gets rendered.
   // c1 through c10 are the containers, the actual test cases are inside.
@@ -270,6 +274,16 @@ function doTest() {
     };
   testAccessibleTree("c15", tree);
 
+  // Test getting descendants of unrendered nodes.
+  ok(!getAccessibleDescendantFor("#c16 > span"),
+     "Span has no accessible children");
+
+  ok(!getAccessibleDescendantFor("#c17 > span"),
+     "Span with relocated child should return null");
+
+  is(getAccessibleDescendantFor("#c12 > div").role, ROLE_PARAGRAPH,
+     "Descendant has correct role")
+
   SimpleTest.finish();
 }
 
@@ -329,5 +343,9 @@ addA11yLoadEvent(doTest);
 
   <!-- Inner div should be rendered because previous sibling is text. -->
   <div id="c15">l1<div></div>l2</div>
+
+  <div id="c16">hello <span></span> world</div>
+
+  <div id="c17"><div aria-owns="c"></div>hello <span><button id="c">b</button></span> world</div>
 </body>
 </html>

accessible/xpcom/xpcAccessibilityService.cpp

@@ -127,6 +127,29 @@ xpcAccessibilityService::GetAccessibleFor(nsINode* aNode,
   return NS_OK;
 }
 
+NS_IMETHODIMP
+xpcAccessibilityService::GetAccessibleDescendantFor(
+    nsINode* aNode, nsIAccessible** aAccessible) {
+  NS_ENSURE_ARG_POINTER(aAccessible);
+  *aAccessible = nullptr;
+  if (!aNode) {
+    return NS_OK;
+  }
+
+  nsAccessibilityService* accService = GetAccService();
+  if (!accService) {
+    return NS_ERROR_SERVICE_NOT_AVAILABLE;
+  }
+
+  DocAccessible* document = accService->GetDocAccessible(aNode->OwnerDoc());
+  if (document) {
+    NS_IF_ADDREF(*aAccessible =
+                     ToXPC(document->GetAccessibleOrDescendant(aNode)));
+  }
+
+  return NS_OK;
+}
+
 NS_IMETHODIMP
 xpcAccessibilityService::GetStringRole(uint32_t aRole, nsAString& aString) {
   nsAccessibilityService* accService = GetAccService();