--- a/layout/base/nsLayoutUtils.cpp
+++ b/layout/base/nsLayoutUtils.cpp
@@ -2055,25 +2055,29 @@ nsLayoutUtils::SetFixedPositionLayerData
     if (position->mOffset.GetRightUnit() != eStyleUnit_Auto) {
       sides |= eSideBitsRight;
       if (position->mOffset.GetLeftUnit() != eStyleUnit_Auto) {
         sides |= eSideBitsLeft;
         anchor.x = anchorRect.x + anchorRect.width / 2.f;
       } else {
         anchor.x = anchorRect.XMost();
       }
+    } else if (position->mOffset.GetLeftUnit() != eStyleUnit_Auto) {
+      sides |= eSideBitsLeft;
     }
     if (position->mOffset.GetBottomUnit() != eStyleUnit_Auto) {
       sides |= eSideBitsBottom;
       if (position->mOffset.GetTopUnit() != eStyleUnit_Auto) {
         sides |= eSideBitsTop;
         anchor.y = anchorRect.y + anchorRect.height / 2.f;
       } else {
         anchor.y = anchorRect.YMost();
       }
+    } else if (position->mOffset.GetTopUnit() != eStyleUnit_Auto) {
+      sides |= eSideBitsTop;
     }
   }
 
   ViewID id = FrameMetrics::NULL_SCROLL_ID;
   if (nsIFrame* rootScrollFrame = aPresContext->PresShell()->GetRootScrollFrame()) {
     if (nsIContent* content = rootScrollFrame->GetContent()) {
       id = FindOrCreateIDFor(content);
     }