Bug 956690 - Ensure we descend into the root scroll frame if there is a displayport set. r=tn

layout/generic/nsFrame.cpp

@@ -2148,7 +2148,13 @@ nsIFrame::BuildDisplayListForChild(nsDisplayListBuilder*   aBuilder,
     // If the child is a scrollframe that we want to ignore, then we need
     // to descend into it because its scrolled child may intersect the dirty
     // area even if the scrollframe itself doesn't.
-    if (child != aBuilder->GetIgnoreScrollFrame()) {
+    // There are cases where the "ignore scroll frame" on the builder is not set
+    // correctly, and so we additionally want to catch cases where the child is
+    // a root scrollframe and we are ignoring scrolling on the viewport.
+    nsIPresShell* shell = PresContext()->PresShell();
+    bool keepDescending = child == aBuilder->GetIgnoreScrollFrame() ||
+        (shell->IgnoringViewportScrolling() && child == shell->GetRootScrollFrame());
+    if (!keepDescending) {
       nsRect childDirty;
       if (!childDirty.IntersectRect(dirty, child->GetVisualOverflowRect()))
         return;