Bug 1191539 - DisplayPort should ForceInside frameRect to match compositor DisplayPort. r=kats

--HG--
extra : commitid : ptd0dqCx1S
extra : rebase_source : fd3e1c04e7c21362c7496d32139ff9f30c2b2d5c

layout/base/nsLayoutUtils.cpp

@@ -920,6 +920,12 @@ GetDisplayPortFromMarginsData(nsIContent* aContent,
   ScreenRect screenRect = LayoutDeviceRect::FromAppUnits(base, auPerDevPixel)
                         * parentRes;
 
+  nsRect expandedScrollableRect =
+    nsLayoutUtils::CalculateExpandedScrollableRect(frame);
+  ScreenRect screenExpScrollableRect =
+    LayoutDeviceRect::FromAppUnits(expandedScrollableRect - scrollPos,
+                                   auPerDevPixel) * parentRes;
+
   if (gfxPrefs::LayersTilesEnabled()) {
     // Note on the correctness of applying the alignment in Screen space:
     //   The correct space to apply the alignment in would be Layer space, but
@@ -943,6 +949,9 @@ GetDisplayPortFromMarginsData(nsIContent* aContent,
     // up to tile boundaries.
     screenRect.Inflate(1);
 
+    // Make sure the displayport remains within the scrollable rect.
+    screenRect = screenRect.ForceInside(screenExpScrollableRect);
+
     // Avoid division by zero.
     if (alignmentX == 0) {
       alignmentX = 1;
@@ -992,6 +1001,9 @@ GetDisplayPortFromMarginsData(nsIContent* aContent,
       screenRect.x -= left;
       screenRect.width += left + right;
     }
+
+    // Make sure the displayport remains within the scrollable rect.
+    screenRect = screenRect.ForceInside(screenExpScrollableRect);
   }
 
   // Convert the aligned rect back into app units.
@@ -1001,7 +1013,6 @@ GetDisplayPortFromMarginsData(nsIContent* aContent,
   result = ApplyRectMultiplier(result, aMultiplier);
 
   // Finally, clamp it to the expanded scrollable rect.
-  nsRect expandedScrollableRect = nsLayoutUtils::CalculateExpandedScrollableRect(frame);
   result = expandedScrollableRect.Intersect(result + scrollPos) - scrollPos;
 
   return result;