--- a/layout/base/nsLayoutDebugger.cpp
+++ b/layout/base/nsLayoutDebugger.cpp
@@ -146,18 +146,18 @@ PrintDisplayItemTo(nsDisplayListBuilder*
   if (aDumpHtml && aItem->Painted()) {
     nsCString string(aItem->Name());
     string.Append('-');
     string.AppendInt((uint64_t)aItem);
     aStream << nsPrintfCString("<a href=\"javascript:ViewImage('%s')\">", string.BeginReading());
   }
 #endif
 
-  aStream << nsPrintfCString("%s p=0x%p f=0x%p(%s) %sbounds(%d,%d,%d,%d) layerBounds(%d,%d,%d,%d) visible(%d,%d,%d,%d) componentAlpha(%d,%d,%d,%d) clip(%s) asr(%s) clipChain(%s)%s ref=0x%p agr=0x%p",
-          aItem->Name(), aItem, (void*)f, NS_ConvertUTF16toUTF8(contentData).get(),
+  aStream << nsPrintfCString("%s p=0x%p f=0x%p(%s) key=%d %sbounds(%d,%d,%d,%d) layerBounds(%d,%d,%d,%d) visible(%d,%d,%d,%d) componentAlpha(%d,%d,%d,%d) clip(%s) asr(%s) clipChain(%s)%s ref=0x%p agr=0x%p",
+          aItem->Name(), aItem, (void*)f, NS_ConvertUTF16toUTF8(contentData).get(), aItem->GetPerFrameKey(),
           (aItem->ZIndex() ? nsPrintfCString("z=%d ", aItem->ZIndex()).get() : ""),
           rect.x, rect.y, rect.width, rect.height,
           layerRect.x, layerRect.y, layerRect.width, layerRect.height,
           vis.x, vis.y, vis.width, vis.height,
           component.x, component.y, component.width, component.height,
           clip.ToString().get(),
           ActiveScrolledRoot::ToString(aItem->GetActiveScrolledRoot()).get(),
           DisplayItemClipChain::ToString(aItem->GetClipChain()).get(),
@@ -185,23 +185,22 @@ PrintDisplayItemTo(nsDisplayListBuilder*
   // Display item specific debug info
   aItem->WriteDebugInfo(aStream);
 
 #ifdef MOZ_DUMP_PAINTING
   if (aDumpHtml && aItem->Painted()) {
     aStream << "</a>";
   }
 #endif
-  uint32_t key = aItem->GetPerFrameKey();
-  Layer* layer = mozilla::FrameLayerBuilder::GetDebugOldLayerFor(f, key);
-  if (layer) {
+  DisplayItemData* data = mozilla::FrameLayerBuilder::GetOldDataFor(aItem);
+  if (data && data->GetLayer()) {
     if (aDumpHtml) {
-      aStream << nsPrintfCString(" <a href=\"#%p\">layer=%p</a>", layer, layer);
+      aStream << nsPrintfCString(" <a href=\"#%p\">layer=%p</a>", data->GetLayer(), data->GetLayer());
     } else {
-      aStream << nsPrintfCString(" layer=0x%p", layer);
+      aStream << nsPrintfCString(" layer=0x%p", data->GetLayer());
     }
   }
 #ifdef MOZ_DUMP_PAINTING
   if (aItem->GetType() == DisplayItemType::TYPE_MASK) {
     nsCString str;
     (static_cast<nsDisplayMask*>(aItem))->PrintEffects(str);
     aStream << str.get();
   }

--- a/layout/painting/FrameLayerBuilder.cpp
+++ b/layout/painting/FrameLayerBuilder.cpp
@@ -2127,26 +2127,26 @@ FrameLayerBuilder::ClearCachedGeometry(n
   nsIFrame* frame = aItem->Frame();
 
   DisplayItemData* oldData = GetOldLayerForFrame(frame, key);
   if (oldData) {
     oldData->mGeometry = nullptr;
   }
 }
 
-/* static */ Layer*
-FrameLayerBuilder::GetDebugOldLayerFor(nsIFrame* aFrame, uint32_t aDisplayItemKey)
+/* static */ DisplayItemData*
+FrameLayerBuilder::GetOldDataFor(nsDisplayItem* aItem)
 {
-  const SmallPointerArray<DisplayItemData>& array = aFrame->DisplayItemData();
+  const SmallPointerArray<DisplayItemData>& array = aItem->Frame()->DisplayItemData();
 
   for (uint32_t i = 0; i < array.Length(); i++) {
     DisplayItemData *data = DisplayItemData::AssertDisplayItemData(array.ElementAt(i));
 
-    if (data->mDisplayItemKey == aDisplayItemKey) {
-      return data->mLayer;
+    if (data->mDisplayItemKey == aItem->GetPerFrameKey()) {
+      return data;
     }
   }
   return nullptr;
 }
 
 // Reset state that should not persist when a layer is recycled.
 static void
 ResetLayerStateForRecycling(Layer* aLayer) {

--- a/layout/painting/FrameLayerBuilder.h
+++ b/layout/painting/FrameLayerBuilder.h
@@ -59,17 +59,17 @@ class ContainerState;
 /**
   * Retained data for a display item.
   */
 class DisplayItemData final {
 public:
   friend class FrameLayerBuilder;
 
   uint32_t GetDisplayItemKey() { return mDisplayItemKey; }
-  layers::Layer* GetLayer() { return mLayer; }
+  layers::Layer* GetLayer() const { return mLayer; }
   nsDisplayItemGeometry* GetGeometry() const { return mGeometry.get(); }
   void Invalidate() { mIsInvalid = true; }
   void ClearAnimationCompositorState();
 
   static DisplayItemData* AssertDisplayItemData(DisplayItemData* aData);
 
   void* operator new(size_t sz, nsPresContext* aPresContext)
   {
@@ -499,17 +499,17 @@ public:
    * frame.
    */
   Layer* GetOldLayerFor(nsDisplayItem* aItem,
                         nsDisplayItemGeometry** aOldGeometry = nullptr,
                         DisplayItemClip** aOldClip = nullptr);
 
   void ClearCachedGeometry(nsDisplayItem* aItem);
 
-  static Layer* GetDebugOldLayerFor(nsIFrame* aFrame, uint32_t aDisplayItemKey);
+  static DisplayItemData* GetOldDataFor(nsDisplayItem* aItem);
 
   /**
    * Return the layer that all display items of aFrame were assigned to in the
    * last paint, or nullptr if there was no single layer assigned to all of the
    * frame's display items (i.e. zero, or more than one).
    * This function is for testing purposes and not performance sensitive.
    */
   template<class T>