Bug 1826194 part 2: When calculating a text equivalent, walk the flat tree instead of only direct children. r=morgan

Previously, when calculating a text equivalent by walking the DOM tree, we only walked direct children of a DOM node.
However, a slot element is a placeholder for content which comes from outside of a shadow root.
In order to include this content when a slot isn't accessible (e.g. because it is hidden), we need to walk the DOM flat tree instead.

Differential Revision: https://phabricator.services.mozilla.com/D181841

accessible/base/nsTextEquivUtils.cpp

@@ -11,6 +11,7 @@
 #include "LocalAccessible-inl.h"
 #include "AccIterator.h"
 #include "nsCoreUtils.h"
+#include "mozilla/dom/ChildIterator.h"
 #include "mozilla/dom/Text.h"
 #include "nsIContentInlines.h"
 
@@ -123,8 +124,9 @@ nsresult nsTextEquivUtils::AppendTextEquivFromTextContent(nsIContent* aContent,
 
 nsresult nsTextEquivUtils::AppendFromDOMChildren(nsIContent* aContent,
                                                  nsAString* aString) {
-  for (nsIContent* childContent = aContent->GetFirstChild(); childContent;
-       childContent = childContent->GetNextSibling()) {
+  auto iter =
+      dom::AllChildrenIterator(aContent, nsIContent::eAllChildren, true);
+  while (nsIContent* childContent = iter.GetNextChild()) {
     nsresult rv = AppendFromDOMNode(childContent, aString);
     NS_ENSURE_SUCCESS(rv, rv);
   }

accessible/tests/mochitest/name/test_general.html

@@ -264,6 +264,8 @@
       const shadowRoot = getNode("shadowHost").shadowRoot;
       const shadowButtonVisibleText = shadowRoot.getElementById("shadowButtonVisibleText");
       testName(shadowButtonVisibleText, "shadowButtonVisibleText");
+      const shadowButtonHiddenText = shadowRoot.getElementById("shadowButtonHiddenText");
+      testName(shadowButtonHiddenText, "shadowButtonHiddenText");
 
       SimpleTest.finish();
     }
@@ -736,10 +738,13 @@
   <!-- aria-labelledby referring to a slot -->
   <div id="shadowHost">
     shadowButtonVisibleText
+    <span slot="hiddenSlot">shadowButtonHiddenText</span>
   </div>
   <template id="shadowTemplate">
     <input type="button" id="shadowButtonVisibleText" aria-labelledby="visibleSlot">
     <slot id="visibleSlot"></slot>
+    <input type="button" id="shadowButtonHiddenText" aria-labelledby="hiddenSlot">
+    <slot id="hiddenSlot" name="hiddenSlot" hidden></slot>
   </template>
   <script>
     const shadowHost = document.getElementById("shadowHost");