Bug 1246443 - Only restrict displayport base to root composition bounds if content has displayport; r=tnikkel

Bug 1241917 made it so that a subframe's displayport base is restricted
to the root composition bounds (in addition to its previous
restrictions). This involved an expensive coordinate transformation
causing a scrolling performance regression.

This avoids restricting the displayport base to the root composition
bounds unless the frame has a display port, avoiding the expensive
computation unless necessary.

MozReview-Commit-ID: FVacUscAfu2

--HG--
extra : transplant_source : %F9%E9%19%06/%9C%EA%8C%D1%D5%BD%ED%26C%97y%15%92%7E%CB

layout/generic/nsGfxScrollFrame.cpp

@@ -3370,6 +3370,9 @@ ScrollFrameHelper::DecideScrollableLayer(nsDisplayListBuilder* aBuilder,
         // scrollport.
         displayportBase = aDirtyRect->Intersect(mScrollPort);
 
+        // Only restrict to the root composition bounds if necessary,
+        // as the required coordinate transformation is expensive.
+        if (wasUsingDisplayPort) {
           const nsPresContext* rootPresContext =
             pc->GetToplevelContentDocumentPresContext();
           if (!rootPresContext) {
@@ -3390,6 +3393,7 @@ ScrollFrameHelper::DecideScrollableLayer(nsDisplayListBuilder* aBuilder,
               displayportBase = displayportBase.Intersect(rootCompBounds);
             }
           }
+        }
 
         displayportBase -= mScrollPort.TopLeft();
       }