Implement accessibility support for CSS-transformed iframes.

third_party/WebKit/Source/modules/accessibility/AXLayoutObject.cpp

Index: third_party/WebKit/Source/modules/accessibility/AXLayoutObject.cpp
diff --git a/third_party/WebKit/Source/modules/accessibility/AXLayoutObject.cpp b/third_party/WebKit/Source/modules/accessibility/AXLayoutObject.cpp
index 3ba0e4eb032618e46fe2f99a28ab31ea29ca25df..5659a9560d9f5a48df708959b57d245c243c2046 100644
--- a/third_party/WebKit/Source/modules/accessibility/AXLayoutObject.cpp
+++ b/third_party/WebKit/Source/modules/accessibility/AXLayoutObject.cpp
@@ -80,6 +80,7 @@
 #include "modules/accessibility/AXSpinButton.h"
 #include "modules/accessibility/AXTable.h"
 #include "platform/fonts/FontTraits.h"
+#include "platform/geometry/TransformState.h"
 #include "platform/text/PlatformLocale.h"
 #include "platform/text/TextDirection.h"
 #include "wtf/StdLibExtras.h"
@@ -218,6 +219,23 @@ LayoutRect AXLayoutObject::elementRect() const
     return m_cachedElementRect;
 }
 
+SkMatrix44 AXLayoutObject::transformFromLocalParentFrame() const
+{
+    if (!m_layoutObject || !documentFrameView() || !documentFrameView()->layoutView())

[dcheng, 2016/03/23 05:26:00]

I'm a little surprised we need the latter two checks: shouldn't having a
LayoutObject imply the existence of the FrameView and LayoutView?

[dmazzoni, 2016/03/24 04:26:53]

What about during tear-down? We've been bitten by bugs before when an
accessibility APi gets called during frame tear-down.

If you're sure it's still impossible I can remove these checks

[dcheng, 2016/03/24 04:33:14]

I guess I don't know enough about how AX hooks in to say this will be true for
sure.

Are there any preconditions around when AXObject::detach() is invoked? Is there
a guarantee that this will happen synchronously when a node is detached from the
DOM?

[dmazzoni, 2016/03/24 04:50:07]

Yes, it happens synchronously, in Node::removedFrom.

Come to think of it, the past problems we had probably all involved weird corner
cases (usually found by clusterfuzz) involving multiple frames. Now that we know
that all of the AXObjects belong to the same document and frame, this ought to
be
a lot more safe.

After reading the code for documentFrameView() and layoutView() I'm comfortable
removing those checks.

+        return SkMatrix44();
+    LayoutView* layoutView = documentFrameView()->layoutView();
+
+    FrameView* parentFrameView = documentFrameView()->parentFrameView();
+    if (!parentFrameView || !parentFrameView->layoutView())

[dcheng, 2016/03/23 05:26:00]

Similar thought about the null check for parentFrameView->layoutView() here.

[dmazzoni, 2016/03/24 04:50:07]

Done.

+        return SkMatrix44();
+    LayoutView* parentLayoutView = parentFrameView->layoutView();
+
+    TransformationMatrix accumulatedTransform = layoutView->localToAncestorTransform(parentLayoutView, TraverseDocumentBoundaries);
+    IntPoint scrollPosition = documentFrameView()->scrollPosition();
+    accumulatedTransform.translate(scrollPosition.x(), scrollPosition.y());
+    return TransformationMatrix::toSkMatrix44(accumulatedTransform);
+}
+
 LayoutBoxModelObject* AXLayoutObject::getLayoutBoxModelObject() const
 {
     if (!m_layoutObject || !m_layoutObject->isBoxModelObject())