drawSystemFocusRing should take canvas transformations into account.

Source/core/html/canvas/CanvasRenderingContext2D.cpp

 /*
  * Copyright (C) 2004, 2005, 2006, 2007, 2008, 2009, 2010, 2011, 2012 Apple Inc. All rights reserved.
  * Copyright (C) 2008, 2010 Nokia Corporation and/or its subsidiary(-ies)
  * Copyright (C) 2007 Alp Toker <alp@atoker.com>
  * Copyright (C) 2008 Eric Seidel <eric@webkit.org>
  * Copyright (C) 2008 Dirk Schulze <krit@webkit.org>
  * Copyright (C) 2010 Torch Mobile (Beijing) Co. Ltd. All rights reserved.
  * Copyright (C) 2012 Intel Corporation. All rights reserved.
  * Copyright (C) 2013 Adobe Systems Incorporated. All rights reserved.
  *
@@ skipping 2367 matching lines @@
 }
 
 void CanvasRenderingContext2D::updateFocusRingAccessibility(const Path& path, Element* element)
 {
     // If accessibility is already enabled in this frame, associate this path's
     // bounding box with the accessible object. Do this even if the element
     // isn't focused because assistive technology might try to explore the object's
     // location before it gets focus.
     if (AXObjectCache* axObjectCache = element->document()->existingAXObjectCache()) {
         if (AccessibilityObject* obj = axObjectCache->getOrCreate(element)) {
+            // Get the bounding rect and apply transformations.
+            FloatRect bounds = m_path.boundingRect();
+            AffineTransform ctm = state().m_transform;
+            FloatRect transformedBounds = ctm.mapRect(bounds);
+            LayoutRect elementRect = LayoutRect(transformedBounds);
+
+            // Offset by the canvas rect and set the bounds of the accessible element.
             IntRect canvasRect = canvas()->renderer()->absoluteBoundingBoxRect();
-            LayoutRect rect = LayoutRect(path.boundingRect());
-            rect.moveBy(canvasRect.location());
-            obj->setElementRect(rect);
+            elementRect.moveBy(canvasRect.location());

[Stephen White, 2013/08/19 18:08:04]

Not new to this patch, but I wonder if this handles weird cases like CSS 3D
transform applied to canvas?

[dmazzoni, 2013/08/20 06:01:32]

I wasn't sure, so I checked with a simple transform and a 3-D transform and it
appeared to work correctly both times!

+            obj->setElementRect(elementRect);
+
+            // Set the bounds of any ancestor accessible elements, up to the canvas element,
+            // otherwise this element will appear to not be within its parent element.
+            obj = obj->parentObject();
+            while (obj && !obj->isCanvas()) {

[Stephen White, 2013/08/19 18:08:04]

Shouldn't we actually compare against this canvas element? What would happen if
we called this API for an element under a different canvas anyway?

[dmazzoni, 2013/08/20 06:01:32]

Agreed, I wasn't thinking about those corner cases. Fixed.

If you called it with the wrong canvas, it should fail in focusRingCallIsValid
because the element isn't a descendant of the canvas.

However, it's possible you could do something silly like put a canvas in a
canvas (yo dawg), in which case it shouldn't crash, it just probably won't work.

+                obj->setElementRect(elementRect);
+                obj = obj->parentObject();
+            }
         }
     }
 }
 
 void CanvasRenderingContext2D::drawFocusRing(const Path& path)
 {
     GraphicsContext* c = drawingContext();
     if (!c)
         return;
 
     c->save();
     c->setAlpha(1.0);
     c->clearShadow();
     c->setCompositeOperation(CompositeSourceOver, BlendModeNormal);
 
     // These should match the style defined in html.css.
     Color focusRingColor = RenderTheme::focusRingColor();
     const int focusRingWidth = 5;
     const int focusRingOutline = 0;
     c->drawFocusRing(path, focusRingWidth, focusRingOutline, focusRingColor);
     didDraw(path.boundingRect());
 
     c->restore();
 }
 
 } // namespace WebCore