Bug 1299715 - Part 3. Clip the target context by visible region of nsDisplayMask. r=mstange

MozReview-Commit-ID: HuqQBdY2M0O

--HG--
extra : rebase_source : c387c0b7745add2bb6bca185d2e7d91734d3e716

layout/base/nsDisplayList.cpp

@@ -6938,9 +6938,18 @@ nsDisplayMask::PaintAsLayer(nsDisplayListBuilder* aBuilder,
                                                   aManager,
                                                   mHandleOpacity);
 
+  // Clip the drawing target by mVisibleRect, which contains the visible
+  // region of the target frame and its out-of-flow and inflow descendants.
+  gfxContext* context = aCtx->ThebesContext();
+  context->Clip(NSRectToSnappedRect(mVisibleRect,
+                                    mFrame->PresContext()->AppUnitsPerDevPixel(),
+                                    *aCtx->GetDrawTarget()));
+
   image::DrawResult result =
     nsSVGIntegrationUtils::PaintMaskAndClipPath(params);
 
+  context->PopClip();
+
   nsDisplaySVGEffectsGeometry::UpdateDrawResult(this, result);
 }