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

@@ -3370,24 +3370,28 @@ ScrollFrameHelper::DecideScrollableLayer(nsDisplayListBuilder* aBuilder,
         // scrollport.
         displayportBase = aDirtyRect->Intersect(mScrollPort);
 
-        const nsPresContext* rootPresContext =
-          pc->GetToplevelContentDocumentPresContext();
-        if (!rootPresContext) {
-          rootPresContext = pc->GetRootPresContext();
-        }
-        if (rootPresContext) {
-          const nsIPresShell* const rootPresShell = rootPresContext->PresShell();
-          nsIFrame* rootFrame = rootPresShell->GetRootScrollFrame();
-          if (!rootFrame) {
-            rootFrame = rootPresShell->GetRootFrame();
+        // 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) {
+            rootPresContext = pc->GetRootPresContext();
           }
-          if (rootFrame) {
-            nsRect rootCompBounds =
-              nsRect(nsPoint(0, 0), nsLayoutUtils::CalculateCompositionSizeForFrame(rootFrame));
+          if (rootPresContext) {
+            const nsIPresShell* const rootPresShell = rootPresContext->PresShell();
+            nsIFrame* rootFrame = rootPresShell->GetRootScrollFrame();
+            if (!rootFrame) {
+              rootFrame = rootPresShell->GetRootFrame();
+            }
+            if (rootFrame) {
+              nsRect rootCompBounds =
+                nsRect(nsPoint(0, 0), nsLayoutUtils::CalculateCompositionSizeForFrame(rootFrame));
 
-            nsLayoutUtils::TransformRect(rootFrame, mOuter, rootCompBounds);
+              nsLayoutUtils::TransformRect(rootFrame, mOuter, rootCompBounds);
 
-            displayportBase = displayportBase.Intersect(rootCompBounds);
+              displayportBase = displayportBase.Intersect(rootCompBounds);
+            }
           }
         }