Fix clippingContainer for fixed position elements

Source/core/rendering/RenderObject.cpp

Index: Source/core/rendering/RenderObject.cpp
diff --git a/Source/core/rendering/RenderObject.cpp b/Source/core/rendering/RenderObject.cpp
index 595041c330ef6cc29bb42dc55922f705bb3a283a..83bc9fbc2669a95b667a05d064ce134958e0634b 100644
--- a/Source/core/rendering/RenderObject.cpp
+++ b/Source/core/rendering/RenderObject.cpp
@@ -815,9 +815,23 @@ RenderBlock* RenderObject::containingBlock() const
     return toRenderBlock(o);
 }
 
-RenderObject* RenderObject::clippingContainer() const
-{
-    for (RenderObject* container = containingBlock(); container; container = container->containingBlock()) {
+RenderObject* RenderObject::clippingContainer()
+{
+    RenderObject* container = this;

[ojan, 2014/02/26 20:55:27]

Had to remove the const for this line. I think I could do some shenanigans with
having a const renderer local variable and a non-const container local variable
to keep the const, but that seemed not worth it.

+    while (container) {
+        if (container->style()->position() == FixedPosition) {
+            for (container = container->parent(); container && !container->canContainFixedPositionObjects(); container = container->parent()) {

[esprehn, 2014/02/26 21:26:02]

Is this right? Now if you run into something that claims it can contain fixed
position objects we'll use the hasClipOrOverflowClip() check below.

If I do <div style="overflow: hidden; transform: translateZ(0)">
  <div style="position: fixed;"></div>
</div>

will the fixed thing really be clipped by the transformed overflow thing?

+                // CSS clip applies to fixed position elements even for ancestors that are not what the
+                // fixed element is positioned with respect to.
+                if (container->hasClip())
+                    return container;
+            }
+        } else {
+            container = container->containingBlock();
+        }
+
+        if (!container)
+            return 0;
         if (container->hasClipOrOverflowClip())
             return container;
     }