diff --git a/layout/generic/nsBlockFrame.cpp b/layout/generic/nsBlockFrame.cpp
index 4b5c5fbb0d52..ec47e9b0daf2 100644
--- a/layout/generic/nsBlockFrame.cpp
+++ b/layout/generic/nsBlockFrame.cpp
@@ -2582,9 +2582,7 @@ nsBlockFrame::SlideLine(nsBlockReflowState& aState,
 
   if (aLine->IsBlock()) {
     if (aDY) {
-      nsPoint p = kid->GetPosition();
-      p.y += aDY;
-      kid->SetPosition(p);
+      kid->MovePositionBy(nsPoint(0, aDY));
     }
 
     // Make sure the frame's view and any child views are updated
@@ -2598,9 +2596,7 @@ nsBlockFrame::SlideLine(nsBlockReflowState& aState,
     int32_t n = aLine->GetChildCount();
     while (--n >= 0) {
       if (aDY) {
-        nsPoint p = kid->GetPosition();
-        p.y += aDY;
-        kid->SetPosition(p);
+        kid->MovePositionBy(nsPoint(0, aDY));
       }
       // Make sure the frame's view and any child views are updated
       ::PlaceFrameView(kid);
diff --git a/layout/generic/nsBlockReflowState.cpp b/layout/generic/nsBlockReflowState.cpp
index c572a3eb5f7d..daafd017b715 100644
--- a/layout/generic/nsBlockReflowState.cpp
+++ b/layout/generic/nsBlockReflowState.cpp
@@ -386,8 +386,7 @@ nsBlockReflowState::RecoverFloats(nsLineList::iterator aLine,
     while (fc) {
       nsIFrame* floatFrame = fc->mFloat;
       if (aDeltaY != 0) {
-        nsPoint p = floatFrame->GetPosition();
-        floatFrame->SetPosition(nsPoint(p.x, p.y + aDeltaY));
+        floatFrame->MovePositionBy(nsPoint(0, aDeltaY));
         nsContainerFrame::PositionFrameView(floatFrame);
         nsContainerFrame::PositionChildViews(floatFrame);
       }
diff --git a/layout/generic/nsFrame.cpp b/layout/generic/nsFrame.cpp
index aa837d3e59d5..0d087536a394 100644
--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -5070,6 +5070,22 @@ ComputeOutlineAndEffectsRect(nsIFrame* aFrame,
   return r;
 }
 
+void
+nsIFrame::MovePositionBy(const nsPoint& aTranslation)
+{
+  nsPoint position = GetNormalPosition() + aTranslation;
+
+  const nsMargin* computedOffsets = nullptr;
+  if (IsRelativelyPositioned()) {
+    computedOffsets = static_cast<nsMargin*>
+      (Properties().Get(nsIFrame::ComputedOffsetProperty()));
+  }
+  nsHTMLReflowState::ApplyRelativePositioning(this, computedOffsets ?
+                                              *computedOffsets : nsMargin(),
+                                              &position);
+  SetPosition(position);
+}
+
 nsPoint
 nsIFrame::GetNormalPosition() const
 {
diff --git a/layout/generic/nsIFrame.h b/layout/generic/nsIFrame.h
index 880557441aba..586eefc43a81 100644
--- a/layout/generic/nsIFrame.h
+++ b/layout/generic/nsIFrame.h
@@ -866,6 +866,13 @@ public:
   }
   void SetPosition(const nsPoint& aPt) { mRect.MoveTo(aPt); }
 
+  /**
+   * Move the frame, accounting for relative positioning. Use this when
+   * adjusting the frame's position by a known amount, to properly update its
+   * saved normal position (see GetNormalPosition below).
+   */
+  void MovePositionBy(const nsPoint& aTranslation);
+
   /**
    * Return frame's position without relative positioning
    */
diff --git a/layout/generic/nsLineLayout.cpp b/layout/generic/nsLineLayout.cpp
index c1b089f9ec69..1fbf0e25880e 100644
--- a/layout/generic/nsLineLayout.cpp
+++ b/layout/generic/nsLineLayout.cpp
@@ -2196,9 +2196,7 @@ nsLineLayout::VerticalAlignFrames(PerSpanData* psd)
 
 static void SlideSpanFrameRect(nsIFrame* aFrame, nscoord aDeltaWidth)
 {
-  nsRect r = aFrame->GetRect();
-  r.x -= aDeltaWidth;
-  aFrame->SetRect(r);
+  aFrame->MovePositionBy(nsPoint(-aDeltaWidth, 0));
 }
 
 bool