Use setSequentialFocusNavigationStartingPoint in ChromeVox

chrome/browser/resources/chromeos/chromevox/cvox2/background/background.js

Index: chrome/browser/resources/chromeos/chromevox/cvox2/background/background.js
diff --git a/chrome/browser/resources/chromeos/chromevox/cvox2/background/background.js b/chrome/browser/resources/chromeos/chromevox/cvox2/background/background.js
index 8b57e089303b6107dd79721e451e75429f5c7e74..48e2ac93ea6fb06bb75eb7a6e1d6046a7534e29e 100644
--- a/chrome/browser/resources/chromeos/chromevox/cvox2/background/background.js
+++ b/chrome/browser/resources/chromeos/chromevox/cvox2/background/background.js
@@ -771,20 +771,37 @@ Background.prototype = {
     // backward.
     var isFocusable = function(node) {
       return node.role != RoleType.iframe &&
+          node.role != RoleType.rootWebArea &&
           node.state.focusable &&
-          !node.state.focused &&
           !AutomationPredicate.structuralContainer(node);
     };
+
+    // First, try to focus the start or end node.
     if (isFocusable(start)) {
-      start.focus();
+      if (!start.state.focused)
+        start.focus();
       return;
-    }
-    if (isFocusable(end)) {
-      end.focus();
+    } else if (isFocusable(end)) {
+      if (!end.state.focused)
+        end.focus();
       return;
     }
 
-    // TODO(dmazzoni): Set sequential focus.
+    // If a common ancestor of |start| and |end| is a link, focus that.
+    var ancestor = AutomationUtil.getLeastCommonAncestor(start, end);
+    while (ancestor && ancestor.root == start.root) {

[David Tseng, 2016/10/27 17:03:18]

Counter example:

<a href="test">
  <button>ok</button>
  <p>with ChromeVox range here call focus on my parent</p>
<button>cancel</button>
  </a>

The correct way to focus here I think is to walk backwards in dfs order until
you find a focusable node. If that focusable node is an ancestor of the current
node/range, then focus it.

[dmazzoni, 2016/10/27 19:42:50]

As discussed in person, NVDA behaves the same for this example
so I don't think it'd be wrong.

As discussed, I'm changing the code to only focus links and
controls. That avoids problems with cases like focusable
toolbars or focusable divs.

+      if (ancestor.role == RoleType.link && ancestor.state.focusable) {
+        if (!ancestor.state.focused)
+          ancestor.focus();
+        return;
+      }
+      ancestor = ancestor.parent;
+    }
+
+    // If nothing is focusable, set the sequential focus navigation starting
+    // point, which ensures that the next time you press Tab, you'll reach
+    // the next or previous focusable node from |start|.
+    start.setSequentialFocusNavigationStartingPoint();
   }
 };