Merge inbound to m-c.

widget/xpwidgets/nsNativeTheme.cpp

@@ -682,17 +682,20 @@ nsNativeTheme::IsRangeHorizontal(nsIFrame* aFrame)
 bool
 nsNativeTheme::IsDarkBackground(nsIFrame* aFrame)
 {
-  nsIScrollableFrame* scrollFrame = aFrame->GetScrollTargetFrame();
+  nsIScrollableFrame* scrollFrame = nullptr;
   while (!scrollFrame && aFrame) {
-    aFrame = aFrame->GetParent();
     scrollFrame = aFrame->GetScrollTargetFrame();
+    aFrame = aFrame->GetParent();
   }
+  if (!scrollFrame)
+    return false;
+
   nsIFrame* frame = scrollFrame->GetScrolledFrame();
   nsStyleContext* bgSC;
   if (nsCSSRendering::FindBackground(frame, &bgSC)) {
     nscolor bgColor = bgSC->StyleBackground()->mBackgroundColor;
     // Consider the background color dark if the sum of the r, g and b values is
-    // less than 384 in a semi-transparent docement.  This heuristic matches what
+    // less than 384 in a semi-transparent document.  This heuristic matches what
     // WebKit does, and we can improve it later if needed.
     return NS_GET_A(bgColor) > 127 &&
            NS_GET_R(bgColor) + NS_GET_G(bgColor) + NS_GET_B(bgColor) < 384;