Bug 764125; Fix a bug where a path with a transform is re-used. r=Bas

gfx/2d/PathCairo.cpp

@@ -44,6 +44,28 @@ CairoPathContext::~CairoPathContext()
   cairo_destroy(mContext);
 }
 
+void
+CairoPathContext::ObserveTarget(DrawTargetCairo* aDrawTarget)
+{
+  if (!aDrawTarget) {
+    return;
+  }
+
+  if (mDrawTarget) {
+    mDrawTarget->SetPathObserver(NULL);
+  }
+  mDrawTarget = aDrawTarget;
+
+  // If there is a transform on the path, then we must have a separate context
+  // from the draw target, so we cannot be its observer
+  if (!mTransform.IsIdentity()) {
+    ForgetDrawTarget();
+    return;
+  }
+
+  mDrawTarget->SetPathObserver(this);
+}
+
 void
 CairoPathContext::DuplicateContextAndPath(const Matrix& aMatrix /* = Matrix() */)
 {
@@ -320,11 +342,7 @@ PathCairo::CopyPathTo(cairo_t* aContext, DrawTargetCairo* aDrawTarget)
 
     // Since aDrawTarget wants us to be the current path on its context, we
     // should also listen to it for updates to that path (as an optimization).
-    // The easiest way to do this is to just recreate mPathContext, since it
-    // registers with aDrawTarget for updates.
-    mPathContext = new CairoPathContext(aContext, aDrawTarget,
-                                        mPathContext->GetFillRule(),
-                                        mPathContext->GetTransform());
+    mPathContext->ObserveTarget(aDrawTarget);
   }
 }
 

gfx/2d/PathCairo.h

@@ -68,6 +68,10 @@ public:
   // Returns true if this CairoPathContext represents path.
   bool ContainsPath(const Path* path);
 
+  // add ourselves as an observer of aDrawTarget, if possible
+  // if we succeed, then mDrawTarget is set to aDrawTarget
+  void ObserveTarget(DrawTargetCairo* aDrawTarget);
+
   cairo_t* GetContext() const { return mContext; }
   DrawTargetCairo* GetDrawTarget() const { return mDrawTarget; }
   Matrix GetTransform() const { return mTransform; }