Canvas2d: Fix incorrect alignment of hit region when applied css/zoom.

third_party/WebKit/Source/modules/canvas2d/EventHitRegion.cpp

 // Copyright 2015 The Chromium Authors. All rights reserved.
 // Use of this source code is governed by a BSD-style license that can be
 // found in the LICENSE file.
 
 #include "modules/canvas2d/EventHitRegion.h"
 
 #include "core/dom/Document.h"
 #include "core/html/HTMLCanvasElement.h"
-#include "core/layout/LayoutObject.h"
+#include "core/layout/LayoutBox.h"
+#include "core/page/Page.h"
 #include "modules/canvas2d/CanvasRenderingContext2D.h"
 #include "modules/canvas2d/HitRegion.h"
 
 namespace blink {
 
 String EventHitRegion::regionIdFromAbsoluteLocation(HTMLCanvasElement& canvas, const LayoutPoint& location)
 {
     CanvasRenderingContext* context = canvas.renderingContext();
     if (!context || !context->is2d())
         return String();
 
     Document& document = canvas.document();
     document.updateLayoutTreeForNodeIfNeeded(&canvas);
 
     // Adjust offsetLocation to be relative to the canvas's position.
-    LayoutObject* layoutObject = canvas.layoutObject();
-    FloatPoint localPos = layoutObject->absoluteToLocal(FloatPoint(location), UseTransforms);
+    LayoutBoxModelObject* lbmo = canvas.layoutBoxModelObject();
+    FloatPoint localPos = lbmo->absoluteToLocal(FloatPoint(location), UseTransforms);
 
-    LocalFrame* frame = document.frame();
-    float zoomFactor = frame ? frame->pageZoomFactor() : 1;
-    float scaleFactor = 1 / zoomFactor;
+    float width = lbmo->offsetWidth();
+    float height = lbmo->offsetHeight();
+    if (lbmo->hasBorderOrPadding()) {
+        float borderAndPaddingLeft = lbmo->borderLeft() + lbmo->paddingLeft();
+        float borderAndPaddingTop = lbmo->borderTop() + lbmo->paddingTop();
+        localPos.move(-borderAndPaddingLeft, -borderAndPaddingTop);
+        width -= lbmo->borderAndPaddingWidth();
+        height -= lbmo->borderAndPaddingHeight();

[Justin Novosad, 2016/01/05 16:48:39]

Isn't there a method that already computes these width and height values for us
already? I would expect LayoutBox::content{Width|Height}() to provide the values
you are looking for.

[zino, 2016/01/06 17:22:31]

Done.

+    }
+    localPos.scale((canvas.width() / width), (canvas.height() / height));
+
+    Page* page = document.page();
+    float scaleFactor = page->deviceScaleFactor();
     if (scaleFactor != 1.0f)
         localPos.scale(scaleFactor, scaleFactor);
 
     HitRegion* hitRegion = toCanvasRenderingContext2D(context)->hitRegionAtPoint(localPos);
     if (!hitRegion || hitRegion->id().isEmpty())
         return String();
 
     return hitRegion->id();
 }
 
 } // namespace blink