A11y: Exposed Hierarchial Level for tablist.

This change exposes hierarchical level for tablist in the accessibility
tree. Hierarchical level is determined from `aria-level` or the DOM
structure of the tablist in relation to its parent tablist.

https://www.w3.org/TR/wai-aria-1.1/#tablist
https://www.w3.org/TR/wai-aria-1.1/#aria-level

Change-Id: I1d6aba1bf47b3998a7622a98ec75dae1e41c9452
Reviewed-on: https://chromium-review.googlesource.com/c/chromium/src/+/1996263
Commit-Queue: Victor Fei <vicfei@microsoft.com>
Reviewed-by: Ian Prest <iapres@microsoft.com>
Reviewed-by: Aaron Leventhal <aleventhal@chromium.org>
Cr-Commit-Position: refs/heads/master@{#738049}

content/browser/accessibility/dump_accessibility_tree_browsertest.cc

@@ -1040,6 +1040,11 @@ IN_PROC_BROWSER_TEST_P(DumpAccessibilityTreeTest, AccessibilityAriaTabList) {
   RunAriaTest(FILE_PATH_LITERAL("aria-tablist.html"));
 }
 
+IN_PROC_BROWSER_TEST_P(DumpAccessibilityTreeTest,
+                       AccessibilityAriaTabListAriaLevel) {
+  RunAriaTest(FILE_PATH_LITERAL("aria-tablist-aria-level.html"));
+}
+
 IN_PROC_BROWSER_TEST_P(DumpAccessibilityTreeTest, AccessibilityAriaTabPanel) {
   RunAriaTest(FILE_PATH_LITERAL("aria-tabpanel.html"));
 }

content/renderer/accessibility/blink_ax_tree_source.cc

@@ -871,10 +871,7 @@ void BlinkAXTreeSource::SerializeNode(WebAXObject src,
     if (ui::IsHeading(dst->role) && src.HeadingLevel()) {
       dst->AddIntAttribute(ax::mojom::IntAttribute::kHierarchicalLevel,
                            src.HeadingLevel());
-    } else if ((dst->role == ax::mojom::Role::kTreeItem ||
-                dst->role == ax::mojom::Role::kComment ||
-                dst->role == ax::mojom::Role::kListItem ||
-                dst->role == ax::mojom::Role::kRow) &&
+    } else if (ui::SupportsHierarchicalLevel(dst->role) &&
                src.HierarchicalLevel()) {
       dst->AddIntAttribute(ax::mojom::IntAttribute::kHierarchicalLevel,
                            src.HierarchicalLevel());

content/test/data/accessibility/aria/aria-tab-expected-blink.txt

 rootWebArea
 ++genericContainer ignored
-++++tabList horizontal setSize=2
+++++tabList horizontal hierarchicalLevel=1 setSize=2
 ++++++tab name='Tab 1' setSize=2 posInSet=1 selected=false
 ++++++tab name='Tab 2' setSize=2 posInSet=2 selected=false

content/test/data/accessibility/aria/aria-tab-expected-win.txt

 ROLE_SYSTEM_DOCUMENT READONLY FOCUSABLE
-++ROLE_SYSTEM_PAGETABLIST IA2_STATE_HORIZONTAL xml-roles:tablist
+++ROLE_SYSTEM_PAGETABLIST IA2_STATE_HORIZONTAL xml-roles:tablist level:1
 ++++ROLE_SYSTEM_PAGETAB name='Tab 1' xml-roles:tab
 ++++ROLE_SYSTEM_PAGETAB name='Tab 2' xml-roles:tab

content/test/data/accessibility/aria/aria-tab.html

@@ -2,11 +2,13 @@
 @MAC-ALLOW:AXRole*
 @WIN-ALLOW:xml-roles*
 @WIN-ALLOW:IA2_STATE_HORIZONTAL
+@WIN-ALLOW:level*
 @AURALINUX-ALLOW:xml-roles*
 @AURALINUX-ALLOW:horizontal
 @AURALINUX-ALLOW:vertical
 @AURALINUX-ALLOW:posinset*
 @AURALINUX-ALLOW:setsize*
+@BLINK-ALLOW:hierarchicalLevel*
 @BLINK-ALLOW:horizontal
 @BLINK-ALLOW:setSize*
 @BLINK-ALLOW:posInSet*

content/test/data/accessibility/aria/aria-tablist-aria-level-expected-android.txt

+android.webkit.WebView focusable focused scrollable
+++android.widget.TabWidget role_description='tab list'
+++++android.view.View role_description='tab' clickable name='Tab 1 of 2, level 1'
+++++android.view.View role_description='tab' clickable name='Tab 2 of 2, level 1' item_index=1 row_index=1
+++android.widget.TabWidget role_description='tab list'
+++++android.view.View role_description='tab' clickable name='Tab 1 of 3, level 2'
+++++android.view.View role_description='tab' clickable name='Tab 2 of 3, level 2' item_index=1 row_index=1
+++++android.widget.TabWidget role_description='tab list'
+++++++android.view.View role_description='tab' clickable name='Tab 1 of 1, level 3'
+++++android.view.View role_description='tab' clickable name='Tab 3 of 3, level 2' item_index=2 row_index=2
+++android.widget.TabWidget role_description='tab list'
+++++android.view.View role_description='tab' clickable name='Tab 1 of 1, level 2'
+++android.widget.TabWidget role_description='tab list'
+++++android.view.View role_description='tab' clickable name='Tab 1 of 1, level 1'

content/test/data/accessibility/aria/aria-tablist-aria-level-expected-auralinux.txt

+[document web]
+++[page tab list] horizontal level:1 setsize:2
+++++[page tab] name='Tab 1 of 2, level 1' selectable posinset:1 setsize:2
+++++[page tab] name='Tab 2 of 2, level 1' selectable posinset:2 setsize:2
+++[page tab list] horizontal level:2 setsize:3
+++++[page tab] name='Tab 1 of 3, level 2' selectable posinset:1 setsize:3
+++++[page tab] name='Tab 2 of 3, level 2' selectable posinset:2 setsize:3
+++++[page tab list] horizontal level:3 setsize:1
+++++++[page tab] name='Tab 1 of 1, level 3' selectable posinset:1 setsize:1
+++++[page tab] name='Tab 3 of 3, level 2' selectable posinset:3 setsize:3
+++[page tab list] horizontal level:2 setsize:1
+++++[page tab] name='Tab 1 of 1, level 2' selectable posinset:1 setsize:1
+++[page tab list] horizontal level:1 setsize:1
+++++[page tab] name='Tab 1 of 1, level 1' selectable posinset:1 setsize:1

content/test/data/accessibility/aria/aria-tablist-aria-level-expected-blink.txt

+rootWebArea
+++genericContainer ignored
+++++tabList horizontal hierarchicalLevel=1 setSize=2
+++++++tab name='Tab 1 of 2, level 1' setSize=2 posInSet=1 selected=false
+++++++tab name='Tab 2 of 2, level 1' setSize=2 posInSet=2 selected=false
+++++tabList horizontal hierarchicalLevel=2 setSize=3
+++++++tab name='Tab 1 of 3, level 2' setSize=3 posInSet=1 selected=false
+++++++tab name='Tab 2 of 3, level 2' setSize=3 posInSet=2 selected=false
+++++++tabList horizontal hierarchicalLevel=3 setSize=1
+++++++++tab name='Tab 1 of 1, level 3' setSize=1 posInSet=1 selected=false
+++++++tab name='Tab 3 of 3, level 2' setSize=3 posInSet=3 selected=false
+++++tabList horizontal hierarchicalLevel=2 setSize=1
+++++++tab name='Tab 1 of 1, level 2' setSize=1 posInSet=1 selected=false
+++++tabList horizontal hierarchicalLevel=1 setSize=1
+++++++tab name='Tab 1 of 1, level 1' setSize=1 posInSet=1 selected=false

content/test/data/accessibility/aria/aria-tablist-aria-level-expected-mac.txt

+AXWebArea AXRoleDescription='HTML content'
+++AXTabGroup AXRoleDescription='tab group'
+++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 1 of 2, level 1' AXValue='0'
+++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 2 of 2, level 1' AXValue='0'
+++AXTabGroup AXRoleDescription='tab group'
+++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 1 of 3, level 2' AXValue='0'
+++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 2 of 3, level 2' AXValue='0'
+++++AXTabGroup AXRoleDescription='tab group'
+++++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 1 of 1, level 3' AXValue='0'
+++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 3 of 3, level 2' AXValue='0'
+++AXTabGroup AXRoleDescription='tab group'
+++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 1 of 1, level 2' AXValue='0'
+++AXTabGroup AXRoleDescription='tab group'
+++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 1 of 1, level 1' AXValue='0'

content/test/data/accessibility/aria/aria-tablist-aria-level-expected-win.txt

+ROLE_SYSTEM_DOCUMENT READONLY FOCUSABLE
+++ROLE_SYSTEM_PAGETABLIST IA2_STATE_HORIZONTAL xml-roles:tablist level:1 setsize:2
+++++ROLE_SYSTEM_PAGETAB name='Tab 1 of 2, level 1' xml-roles:tab setsize:2 posinset:1
+++++ROLE_SYSTEM_PAGETAB name='Tab 2 of 2, level 1' xml-roles:tab setsize:2 posinset:2
+++ROLE_SYSTEM_PAGETABLIST IA2_STATE_HORIZONTAL xml-roles:tablist level:2 setsize:3
+++++ROLE_SYSTEM_PAGETAB name='Tab 1 of 3, level 2' xml-roles:tab setsize:3 posinset:1
+++++ROLE_SYSTEM_PAGETAB name='Tab 2 of 3, level 2' xml-roles:tab setsize:3 posinset:2
+++++ROLE_SYSTEM_PAGETABLIST IA2_STATE_HORIZONTAL xml-roles:tablist level:3 setsize:1
+++++++ROLE_SYSTEM_PAGETAB name='Tab 1 of 1, level 3' xml-roles:tab setsize:1 posinset:1
+++++ROLE_SYSTEM_PAGETAB name='Tab 3 of 3, level 2' xml-roles:tab setsize:3 posinset:3
+++ROLE_SYSTEM_PAGETABLIST IA2_STATE_HORIZONTAL xml-roles:tablist level:2 setsize:1
+++++ROLE_SYSTEM_PAGETAB name='Tab 1 of 1, level 2' xml-roles:tab setsize:1 posinset:1
+++ROLE_SYSTEM_PAGETABLIST IA2_STATE_HORIZONTAL xml-roles:tablist level:1 setsize:1
+++++ROLE_SYSTEM_PAGETAB name='Tab 1 of 1, level 1' xml-roles:tab setsize:1 posinset:1

content/test/data/accessibility/aria/aria-tablist-aria-level.html

+<!--
+@MAC-ALLOW:AXRole*
+@WIN-ALLOW:xml-roles*
+@WIN-ALLOW:level*
+@WIN-ALLOW:setsize*
+@WIN-ALLOW:posinset*
+@AURALINUX-ALLOW:level:*
+@AURALINUX-ALLOW:posinset*
+@AURALINUX-ALLOW:setsize*
+@BLINK-ALLOW:hierarchicalLevel*
+@BLINK-ALLOW:posInSet*
+@BLINK-ALLOW:setSize*
+-->
+<!DOCTYPE html>
+<html>
+<body>
+  <div role="tablist" aria-level="1">
+    <div role="tab">Tab 1 of 2, level 1</div>
+    <div role="tab">Tab 2 of 2, level 1</div>
+  </div>
+  <div role="tablist" aria-level="2">
+    <div role="tab">Tab 1 of 3, level 2</div>
+    <div role="tab">Tab 2 of 3, level 2</div>
+    <div role="tablist" aria-level="3">
+      <div role="tab">Tab 1 of 1, level 3</div>
+    </div>
+    <div role="tab">Tab 3 of 3, level 2</div>
+  </div>
+  <div role="tablist" aria-level="2">
+    <div role="tab">Tab 1 of 1, level 2</div>
+  </div>
+  <div role="tablist" aria-level="1">
+    <div role="tab">Tab 1 of 1, level 1</div>
+  </div>
+</body>
+
+</html>

content/test/data/accessibility/aria/aria-tablist-expected-android.txt

 android.webkit.WebView focusable focused scrollable
 ++android.widget.TabWidget role_description='tab list'
-++++android.view.View role_description='tab' clickable name='Tab 1'
-++++android.view.View role_description='tab' clickable name='Tab 2' item_index=1 row_index=1
+++++android.view.View role_description='tab' clickable name='Tab 1, level 1'
+++++android.view.View role_description='tab' clickable name='Tab 2, level 1' item_index=1 row_index=1
+++++android.widget.TabWidget role_description='tab list'
+++++++android.view.View role_description='tab' clickable name='Tab 1, level 2'
+++++++android.view.View role_description='tab' clickable name='Tab 2, level 2' item_index=1 row_index=1
+++++++++android.widget.TextView name='Tab 2, level 2'
+++++++++android.widget.TabWidget role_description='tab list'
+++++++++++android.view.View role_description='tab' clickable name='Tab 1, level 3'
+++++android.view.View role_description='tab' clickable name='Tab 3, level 1' item_index=2 row_index=2

content/test/data/accessibility/aria/aria-tablist-expected-auralinux.txt

 [document web]
-++[page tab list] horizontal setsize:2
-++++[page tab] name='Tab 1' selectable posinset:1 setsize:2
-++++[page tab] name='Tab 2' selectable posinset:2 setsize:2
+++[page tab list] horizontal setsize:3
+++++[page tab] name='Tab 1, level 1' selectable posinset:1 setsize:3
+++++[page tab] name='Tab 2, level 1' selectable posinset:2 setsize:3
+++++[page tab list] horizontal setsize:2
+++++++[page tab] name='Tab 1, level 2' selectable posinset:1 setsize:2
+++++++[page tab] name='Tab 2, level 2' selectable posinset:2 setsize:2
+++++++++[static] name='Tab 2, level 2'
+++++++++[page tab list] horizontal setsize:1
+++++++++++[page tab] name='Tab 1, level 3' selectable posinset:1 setsize:1
+++++[page tab] name='Tab 3, level 1' selectable posinset:3 setsize:3

content/test/data/accessibility/aria/aria-tablist-expected-blink.txt

 rootWebArea
 ++genericContainer ignored
-++++tabList horizontal
-++++++tab name='Tab 1' selected=false
-++++++tab name='Tab 2' selected=false
+++++tabList horizontal hierarchicalLevel=1 setSize=3
+++++++tab name='Tab 1, level 1' setSize=3 posInSet=1 selected=false
+++++++tab name='Tab 2, level 1' setSize=3 posInSet=2 selected=false
+++++++tabList horizontal hierarchicalLevel=2 setSize=2
+++++++++tab name='Tab 1, level 2' setSize=2 posInSet=1 selected=false
+++++++++tab name='Tab 2, level 2' setSize=2 posInSet=2 selected=false
+++++++++++staticText name='Tab 2, level 2'
+++++++++++++inlineTextBox name='Tab 2, level 2'
+++++++++++tabList horizontal hierarchicalLevel=3 setSize=1
+++++++++++++tab name='Tab 1, level 3' setSize=1 posInSet=1 selected=false
+++++++tab name='Tab 3, level 1' setSize=3 posInSet=3 selected=false
+<-- End-of-file -->

content/test/data/accessibility/aria/aria-tablist-expected-mac.txt

 AXWebArea AXRoleDescription='HTML content'
 ++AXTabGroup AXRoleDescription='tab group'
-++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 1' AXValue='0'
-++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 2' AXValue='0'
+++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 1, level 1' AXValue='0'
+++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 2, level 1' AXValue='0'
+++++AXTabGroup AXRoleDescription='tab group'
+++++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 1, level 2' AXValue='0'
+++++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 2, level 2' AXValue='0'
+++++++++AXStaticText AXRoleDescription='text' AXValue='Tab 2, level 2'
+++++++++AXTabGroup AXRoleDescription='tab group'
+++++++++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 1, level 3' AXValue='0'
+++++AXRadioButton AXRoleDescription='tab' AXTitle='Tab 3, level 1' AXValue='0'

content/test/data/accessibility/aria/aria-tablist-expected-win.txt

 ROLE_SYSTEM_DOCUMENT READONLY FOCUSABLE
-++ROLE_SYSTEM_PAGETABLIST IA2_STATE_HORIZONTAL xml-roles:tablist
-++++ROLE_SYSTEM_PAGETAB name='Tab 1' xml-roles:tab
-++++ROLE_SYSTEM_PAGETAB name='Tab 2' xml-roles:tab
+++ROLE_SYSTEM_PAGETABLIST IA2_STATE_HORIZONTAL xml-roles:tablist level:1
+++++ROLE_SYSTEM_PAGETAB name='Tab 1, level 1' xml-roles:tab
+++++ROLE_SYSTEM_PAGETAB name='Tab 2, level 1' xml-roles:tab
+++++ROLE_SYSTEM_PAGETABLIST IA2_STATE_HORIZONTAL xml-roles:tablist level:2
+++++++ROLE_SYSTEM_PAGETAB name='Tab 1, level 2' xml-roles:tab
+++++++ROLE_SYSTEM_PAGETAB name='Tab 2, level 2' xml-roles:tab
+++++++++ROLE_SYSTEM_STATICTEXT name='Tab 2, level 2'
+++++++++ROLE_SYSTEM_PAGETABLIST IA2_STATE_HORIZONTAL xml-roles:tablist level:3
+++++++++++ROLE_SYSTEM_PAGETAB name='Tab 1, level 3' xml-roles:tab
+++++ROLE_SYSTEM_PAGETAB name='Tab 3, level 1' xml-roles:tab
+<-- End-of-file -->

content/test/data/accessibility/aria/aria-tablist.html

@@ -2,18 +2,37 @@
 @MAC-ALLOW:AXRole*
 @WIN-ALLOW:xml-roles*
 @WIN-ALLOW:IA2_STATE_HORIZONTAL
+@WIN-ALLOW:level*
+@BLINK-ALLOW:hierarchicalLevel*
 @BLINK-ALLOW:horizontal
 @AURALINUX-ALLOW:horizontal
 @AURALINUX-ALLOW:vertical
 @AURALINUX-ALLOW:posinset*
 @AURALINUX-ALLOW:setsize*
+@BLINK-ALLOW:setSize*
+@BLINK-ALLOW:posInSet*
 -->
 <!DOCTYPE html>
 <html>
 <body>
   <div role="tablist">
-    <div role="tab" id="tab1">Tab 1</div>
-    <div role="tab" id="tab2">Tab 2</div>
+    <div role="tab">Tab 1, level 1</div>
+    <div role="tab">Tab 2, level 1</div>
+    <ul role="tablist">
+      <li role="tab">Tab 1, level 2</li>
+      <li role="tab">Tab 2, level 2
+        <ul role="tablist">
+          <li role="tab">Tab 1, level 3</li>
+        </ul>
+      </li>
+    </ul>
+    <div role="tab">Tab 3, level 1</div>
   </div>
+
+  <style>
+    li {
+      list-style-type: none;
+    }
+  </style>
 </body>
 </html>

content/test/data/accessibility/event/aria-multiselectable-changed-expected-win.txt

 EVENT_OBJECT_STATECHANGE on <div> role=ROLE_SYSTEM_LIST MULTISELECTABLE,EXTSELECTABLE IA2_STATE_VERTICAL
 EVENT_OBJECT_STATECHANGE on <div> role=ROLE_SYSTEM_OUTLINE
 EVENT_OBJECT_STATECHANGE on <div> role=ROLE_SYSTEM_OUTLINE IA2_STATE_VERTICAL
-EVENT_OBJECT_STATECHANGE on <div> role=ROLE_SYSTEM_PAGETABLIST MULTISELECTABLE,EXTSELECTABLE IA2_STATE_HORIZONTAL
+EVENT_OBJECT_STATECHANGE on <div> role=ROLE_SYSTEM_PAGETABLIST MULTISELECTABLE,EXTSELECTABLE IA2_STATE_HORIZONTAL level=1
 EVENT_OBJECT_STATECHANGE on <div> role=ROLE_SYSTEM_TABLE MULTISELECTABLE,EXTSELECTABLE

third_party/blink/renderer/modules/accessibility/ax_node_object.cc

@@ -1506,44 +1506,47 @@ unsigned AXNodeObject::HierarchicalLevel() const {
       return level;
   }
 
-  // Only tree item and list item will calculate its level through the DOM
-  // currently.
-  if (RoleValue() == ax::mojom::Role::kTreeItem) {
-    // Hierarchy leveling starts at 1, to match the aria-level spec.
-    // We measure tree hierarchy by the number of groups that the item is
-    // within.
-    level = 1;
+  // Helper lambda for calculating hierarchical levels by counting ancestor
+  // nodes that match a target role.
+  auto accumulateLevel = [&](int initial_level, ax::mojom::Role target_role) {
+    int level = initial_level;
     for (AXObject* parent = ParentObject(); parent;
          parent = parent->ParentObject()) {
-      ax::mojom::Role parent_role = parent->RoleValue();
-      if (parent_role == ax::mojom::Role::kGroup)
+      if (parent->RoleValue() == target_role)
         level++;
-      else if (parent_role == ax::mojom::Role::kTree)
-        break;
     }
     return level;
-  } else if (RoleValue() == ax::mojom::Role::kListItem) {
-    level = 0;
-    for (AXObject* parent = ParentObject(); parent;
-         parent = parent->ParentObject()) {
-      ax::mojom::Role parent_role = parent->RoleValue();
-      if (parent_role == ax::mojom::Role::kList)
-        level++;
-    }
+  };
 
-    // When level count is 0 due to this list item not having an ancestor of
-    // Role::kList, not nested in list groups, this list item has a level of 1.
-    return level == 0 ? 1 : level;
-  } else if (RoleValue() == ax::mojom::Role::kComment) {
-    // Comment: level is based on counting comment ancestors until the root.
-    level = 1;
-    for (AXObject* parent = ParentObject(); parent;
-         parent = parent->ParentObject()) {
-      ax::mojom::Role parent_role = parent->RoleValue();
-      if (parent_role == ax::mojom::Role::kComment)
-        level++;
+  switch (RoleValue()) {
+    case ax::mojom::Role::kComment:
+      // Comment: level is based on counting comment ancestors until the root.
+      return accumulateLevel(1, ax::mojom::Role::kComment);
+    case ax::mojom::Role::kListItem:
+      level = accumulateLevel(0, ax::mojom::Role::kList);
+      // When level count is 0 due to this list item not having an ancestor of
+      // Role::kList, not nested in list groups, this list item has a level
+      // of 1.
+      return level == 0 ? 1 : level;
+    case ax::mojom::Role::kTabList:
+      return accumulateLevel(1, ax::mojom::Role::kTabList);
+    case ax::mojom::Role::kTreeItem: {
+      // Hierarchy leveling starts at 1, to match the aria-level spec.
+      // We measure tree hierarchy by the number of groups that the item is
+      // within.
+      level = 1;
+      for (AXObject* parent = ParentObject(); parent;
+           parent = parent->ParentObject()) {
+        ax::mojom::Role parent_role = parent->RoleValue();
+        if (parent_role == ax::mojom::Role::kGroup)
+          level++;
+        else if (parent_role == ax::mojom::Role::kTree)
+          break;
+      }
+      return level;
     }
-    return level;
+    default:
+      return 0;
   }
 
   return 0;

ui/accessibility/ax_role_properties.cc

@@ -617,6 +617,19 @@ bool SupportsExpandCollapse(const ax::mojom::Role role) {
   }
 }
 
+bool SupportsHierarchicalLevel(const ax::mojom::Role role) {
+  switch (role) {
+    case ax::mojom::Role::kComment:
+    case ax::mojom::Role::kListItem:
+    case ax::mojom::Role::kRow:
+    case ax::mojom::Role::kTabList:
+    case ax::mojom::Role::kTreeItem:
+      return true;
+    default:
+      return false;
+  }
+}
+
 bool SupportsOrientation(const ax::mojom::Role role) {
   switch (role) {
     case ax::mojom::Role::kComboBoxGrouping:

ui/accessibility/ax_role_properties.h

@@ -133,6 +133,9 @@ AX_EXPORT bool IsTextOrLineBreak(ax::mojom::Role role);
 // Returns true if the role supports expand/collapse.
 AX_EXPORT bool SupportsExpandCollapse(const ax::mojom::Role role);
 
+// Returns true if the role supports hierarchical level.
+AX_EXPORT bool SupportsHierarchicalLevel(const ax::mojom::Role role);
+
 // Returns true if the provided role can have an orientation.
 AX_EXPORT bool SupportsOrientation(const ax::mojom::Role role);
 

ui/accessibility/ax_role_properties_unittest.cc

@@ -12,6 +12,34 @@
 
 namespace ui {
 
+TEST(AXRolePropertiesTest, TestSupportsHierarchicalLevel) {
+  // Test for iterating through all roles and validate if a role supports
+  // hierarchical level.
+  std::unordered_set<ax::mojom::Role>
+      roles_expected_supports_hierarchical_level = {
+          ax::mojom::Role::kComment, ax::mojom::Role::kListItem,
+          ax::mojom::Role::kRow, ax::mojom::Role::kTabList,
+          ax::mojom::Role::kTreeItem};
+
+  for (int role_idx = static_cast<int>(ax::mojom::Role::kMinValue);
+       role_idx <= static_cast<int>(ax::mojom::Role::kMaxValue); role_idx++) {
+    ax::mojom::Role role = static_cast<ax::mojom::Role>(role_idx);
+    bool supports_hierarchical_level = SupportsHierarchicalLevel(role);
+
+    SCOPED_TRACE(testing::Message() << "ax::mojom::Role=" << ToString(role)
+                                    << ", Actual: supportsHierarchicalLevel="
+                                    << supports_hierarchical_level
+                                    << ", Expected: supportsHierarchicalLevel="
+                                    << !supports_hierarchical_level);
+
+    if (roles_expected_supports_hierarchical_level.find(role) !=
+        roles_expected_supports_hierarchical_level.end())
+      EXPECT_TRUE(supports_hierarchical_level);
+    else
+      EXPECT_FALSE(supports_hierarchical_level);
+  }
+}
+
 TEST(AXRolePropertiesTest, TestSupportsToggle) {
   // Test for iterating through all roles and validate if a role supports
   // toggle.
@@ -36,5 +64,4 @@ TEST(AXRolePropertiesTest, TestSupportsToggle) {
       EXPECT_FALSE(supports_toggle);
   }
 }
-
 }  // namespace ui

ui/accessibility/ax_tree.cc

@@ -1914,7 +1914,15 @@ void AXTree::RecursivelyPopulateOrderedSetItems(
       // If the hierarchical level of |child| and the level of |original_node|
       // differ, we do not add child to |items_to_be_populated| and we do not
       // recurse into |child| and populate its order set item descendants.
-      if (child_level != original_node_min_level) {
+      // Additionally, as an exception, we always add tab items to the set,
+      // because according to WAI-ARIA spec, tab does not support hierarchical
+      // level, while tab's set container tablist supports hierarchical level.
+      // Due to this, we always assume sibling tabs are always on the same
+      // level, and always add tab child item to |items_to_be_populated|.
+      // https://www.w3.org/WAI/PF/aria/roles#tab
+      // https://www.w3.org/WAI/PF/aria/roles#tablist
+      if (child_level != original_node_min_level &&
+          child->data().role != ax::mojom::Role::kTab) {
         if (child_level < original_node_min_level &&
             original_node.GetUnignoredParent() == child->GetUnignoredParent()) {
           // For a flattened structure, where |original_node| and |child| share