--- a/layout/generic/nsAbsoluteContainingBlock.cpp
+++ b/layout/generic/nsAbsoluteContainingBlock.cpp
@@ -113,26 +113,18 @@ nsAbsoluteContainingBlock::Reflow(nsCont
                                   const nsHTMLReflowState& aReflowState,
                                   nsReflowStatus&          aReflowStatus,
                                   const nsRect&            aContainingBlock,
                                   AbsPosReflowFlags        aFlags,
                                   nsOverflowAreas*         aOverflowAreas)
 {
   nsReflowStatus reflowStatus = NS_FRAME_COMPLETE;
 
-  bool reflowAll = aReflowState.ShouldReflowAllKids();
-
-  // The 'width' check below is an optimization to avoid the virtual GetType()
-  // call in most cases.  'aContainingBlock' isn't used for grid items,
-  // each item has its own CB on a frame property instead.
-  // @see nsGridContainerFrame::ReflowChildren
-  const bool isGrid =
-    aContainingBlock.width == nsGridContainerFrame::VERY_LIKELY_A_GRID_CONTAINER &&
-    aDelegatingFrame->GetType() == nsGkAtoms::gridContainerFrame;
-
+  const bool reflowAll = aReflowState.ShouldReflowAllKids();
+  const bool isGrid = !!(aFlags & AbsPosReflowFlags::eIsGridContainerCB);
   nsIFrame* kidFrame;
   nsOverflowContinuationTracker tracker(aDelegatingFrame, true);
   for (kidFrame = mAbsoluteFrames.FirstChild(); kidFrame; kidFrame = kidFrame->GetNextSibling()) {
     bool kidNeedsReflow = reflowAll || NS_SUBTREE_DIRTY(kidFrame) ||
       FrameDependsOnContainer(kidFrame,
                               !!(aFlags & AbsPosReflowFlags::eCBWidthChanged),
                               !!(aFlags & AbsPosReflowFlags::eCBHeightChanged));
     if (kidNeedsReflow && !aPresContext->HasPendingInterrupt()) {

--- a/layout/generic/nsAbsoluteContainingBlock.h
+++ b/layout/generic/nsAbsoluteContainingBlock.h
@@ -65,20 +65,21 @@ public:
                     ChildListID    aListID,
                     nsIFrame*      aPrevFrame,
                     nsFrameList&   aFrameList);
   void RemoveFrame(nsIFrame*      aDelegatingFrame,
                    ChildListID    aListID,
                    nsIFrame*      aOldFrame);
 
   enum class AbsPosReflowFlags {
-    eConstrainHeight = 0x1,
-    eCBWidthChanged  = 0x2,
-    eCBHeightChanged = 0x4,
+    eConstrainHeight   = 0x1,
+    eCBWidthChanged    = 0x2,
+    eCBHeightChanged   = 0x4,
     eCBWidthAndHeightChanged = eCBWidthChanged | eCBHeightChanged,
+    eIsGridContainerCB = 0x8,
   };
 
   /**
    * Called by the delegating frame after it has done its reflow first. This
    * function will reflow any absolutely positioned child frames that need to
    * be reflowed, e.g., because the absolutely positioned child frame has
    * 'auto' for an offset, or a percentage based width or height.
    *

--- a/layout/generic/nsGridContainerFrame.cpp
+++ b/layout/generic/nsGridContainerFrame.cpp
@@ -3239,23 +3239,24 @@ nsGridContainerFrame::ReflowChildren(Gri
         nsRect* cb = static_cast<nsRect*>(child->Properties().Get(
                        GridItemContainingBlockRect()));
         if (!cb) {
           cb = new nsRect;
           child->Properties().Set(GridItemContainingBlockRect(), cb);
         }
         *cb = itemCB.GetPhysicalRect(wm, gridCBPhysicalSize);
       }
-      // This rect isn't used at all for layout so we use it to optimize
-      // away the virtual GetType() call in the callee in most cases.
-      // @see nsAbsoluteContainingBlock::Reflow
-      nsRect dummyRect(0, 0, VERY_LIKELY_A_GRID_CONTAINER, 0);
+      // We pass a dummy rect as CB because each child has its own CB rect.
+      // The eIsGridContainerCB flag tells nsAbsoluteContainingBlock::Reflow to
+      // use those instead.
+      nsRect dummyRect;
       AbsPosReflowFlags flags =
         AbsPosReflowFlags::eCBWidthAndHeightChanged; // XXX could be optimized
       flags |= AbsPosReflowFlags::eConstrainHeight;
+      flags |= AbsPosReflowFlags::eIsGridContainerCB;
       GetAbsoluteContainingBlock()->Reflow(this, pc, *aState.mReflowState,
                                            aStatus, dummyRect, flags,
                                            &aDesiredSize.mOverflowAreas);
     }
   }
 }
 
 void

--- a/layout/generic/nsGridContainerFrame.h
+++ b/layout/generic/nsGridContainerFrame.h
@@ -81,19 +81,16 @@ public:
     };
 
     nscoord mBase;
     nscoord mLimit;
     nscoord mPosition;  // zero until we apply 'align/justify-content'
     StateBits mState;
   };
 
-  // @see nsAbsoluteContainingBlock::Reflow about this magic number
-  static const nscoord VERY_LIKELY_A_GRID_CONTAINER = -123456789;
-
   NS_DECLARE_FRAME_PROPERTY(GridItemContainingBlockRect, DeleteValue<nsRect>)
 
   NS_DECLARE_FRAME_PROPERTY(GridColTrackSizes, DeleteValue<nsTArray<nscoord>>)
 
   const nsTArray<nscoord>* GetComputedTemplateColumns()
   {
     return static_cast<nsTArray<nscoord>*>(Properties().Get(GridColTrackSizes()));
   }