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

layout/base/nsLayoutUtils.cpp

@@ -920,6 +920,9 @@ GetDisplayPortFromMarginsData(nsIContent* aContent,
   ScreenRect screenRect = LayoutDeviceRect::FromAppUnits(base, auPerDevPixel)
                         * parentRes;
 
+  nsRect expandedScrollableRect =
+    nsLayoutUtils::CalculateExpandedScrollableRect(frame);
+
   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
@@ -955,12 +958,20 @@ GetDisplayPortFromMarginsData(nsIContent* aContent,
                                 * res;
 
     screenRect += scrollPosScreen;
+    // Round-out the display port to the nearest alignment (tiles)
     float x = alignmentX * floor(screenRect.x / alignmentX);
     float y = alignmentY * floor(screenRect.y / alignmentY);
-    float w = alignmentX * ceil(screenRect.XMost() / alignmentX) - x;
-    float h = alignmentY * ceil(screenRect.YMost() / alignmentY) - y;
+    float w = alignmentX * ceil(screenRect.width / alignmentX + 1);
+    float h = alignmentY * ceil(screenRect.height / alignmentY + 1);
     screenRect = ScreenRect(x, y, w, h);
     screenRect -= scrollPosScreen;
+
+    ScreenRect screenExpScrollableRect =
+      LayoutDeviceRect::FromAppUnits(expandedScrollableRect,
+                                     auPerDevPixel) * res;
+
+    // Make sure the displayport remains within the scrollable rect.
+    screenRect = screenRect.ForceInside(screenExpScrollableRect - scrollPosScreen);
   } else {
     nscoord maxSizeInAppUnits = GetMaxDisplayPortSize(aContent);
     if (maxSizeInAppUnits == nscoord_MAX) {
@@ -1001,7 +1012,6 @@ 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;