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

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

Index: third_party/WebKit/Source/modules/canvas2d/EventHitRegion.cpp
diff --git a/third_party/WebKit/Source/modules/canvas2d/EventHitRegion.cpp b/third_party/WebKit/Source/modules/canvas2d/EventHitRegion.cpp
index fe7393ff5208476568d58103af3c2c92aed56df5..647710e09626e89867ac61957967d185f41fe9c6 100644
--- a/third_party/WebKit/Source/modules/canvas2d/EventHitRegion.cpp
+++ b/third_party/WebKit/Source/modules/canvas2d/EventHitRegion.cpp
@@ -6,7 +6,8 @@
 
 #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"
 
@@ -22,12 +23,22 @@ String EventHitRegion::regionIdFromAbsoluteLocation(HTMLCanvasElement& canvas, c
     document.updateLayoutTreeForNodeIfNeeded(&canvas);
 
     // Adjust offsetLocation to be relative to the canvas's position.
-    LayoutObject* layoutObject = canvas.layoutObject();
-    FloatPoint localPos = layoutObject->absoluteToLocal(FloatPoint(location), UseTransforms);
-
-    LocalFrame* frame = document.frame();
-    float zoomFactor = frame ? frame->pageZoomFactor() : 1;
-    float scaleFactor = 1 / zoomFactor;
+    LayoutBoxModelObject* lbmo = canvas.layoutBoxModelObject();
+    FloatPoint localPos = lbmo->absoluteToLocal(FloatPoint(location), UseTransforms);
+
+    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);