--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -352,27 +352,16 @@ public:
   }
   nscoord GetMarginSizeInCrossAxis(const LogicalMargin& aMargin) const {
     // If we're row-oriented, our cross axis is the block axis.
     return IsRowOriented()
       ? aMargin.BStartEnd(mWM)
       : aMargin.IStartEnd(mWM);
   }
 
-  // Returns aFrame's computed value for 'height' or 'width' -- whichever is in
-  // the cross-axis. (NOTE: This is cross-axis-specific for now. If we need a
-  // main-axis version as well, we could generalize or clone this function.)
-  const nsStyleCoord& ComputedCrossSize(const nsIFrame* aFrame) const {
-    const nsStylePosition* stylePos = aFrame->StylePosition();
-
-    return IsCrossAxisHorizontal() ?
-      stylePos->mWidth :
-      stylePos->mHeight;
-  }
-
   /**
    * Converts a "flex-relative" point (a main-axis & cross-axis coordinate)
    * into a LogicalPoint, using the flex container's writing mode.
    *
    *  @arg aMainCoord  The main-axis coordinate -- i.e an offset from the
    *                   main-start edge of the flex container's content box.
    *  @arg aCrossCoord The cross-axis coordinate -- i.e an offset from the
    *                   cross-start edge of the flex container's content box.
@@ -551,16 +540,19 @@ public:
 
   bool HadMinViolation() const     { return mHadMinViolation; }
   bool HadMaxViolation() const     { return mHadMaxViolation; }
 
   // Indicates whether this item received a preliminary "measuring" reflow
   // before its actual reflow.
   bool HadMeasuringReflow() const  { return mHadMeasuringReflow; }
 
+  // Indicates whether this item's computed cross-size property is 'auto'.
+  bool IsCrossSizeAuto() const;
+
   // Indicates whether this item's cross-size has been stretched (from having
   // "align-self: stretch" with an auto cross-size and no auto margins in the
   // cross axis).
   bool IsStretched() const         { return mIsStretched; }
 
   // Indicates whether we need to resolve an 'auto' value for the main-axis
   // min-[width|height] property.
   bool NeedsMinSizeAutoResolution() const
@@ -1995,16 +1987,28 @@ FlexItem::GetBaselineOffsetFromOuterCros
              "we're not using the top side, so that only leaves the bottom...");
 
   // Measuring from bottom: The distance from the margin-box bottom edge to the
   // baseline is just the margin-box cross size (i.e. outer cross size), minus
   // the already-computed distance from margin-top to baseline.
   return GetOuterCrossSize(crossAxis) - marginTopToBaseline;
 }
 
+bool
+FlexItem::IsCrossSizeAuto() const
+{
+  const nsStylePosition* stylePos = mFrame->StylePosition();
+  // Check whichever component is in the flex container's cross axis.
+  // (IsInlineAxisCrossAxis() tells us whether that's our ISize or BSize, in
+  // terms of our own WritingMode, mWM.)
+  return eStyleUnit_Auto == (IsInlineAxisCrossAxis()
+                             ? stylePos->ISize(mWM).GetUnit()
+                             : stylePos->BSize(mWM).GetUnit());
+}
+
 uint32_t
 FlexItem::GetNumAutoMarginsInAxis(AxisOrientationType aAxis) const
 {
   uint32_t numAutoMargins = 0;
   const nsStyleSides& styleMargin = mFrame->StyleMargin()->mMargin;
   for (uint32_t i = 0; i < eNumAxisEdges; i++) {
     mozilla::Side side = kAxisOrientationToSidesMap[aAxis][i];
     if (styleMargin.GetUnit(side) == eStyleUnit_Auto) {
@@ -3275,17 +3279,17 @@ FlexItem::ResolveStretchedCrossSize(nsco
                                     const FlexboxAxisTracker& aAxisTracker)
 {
   AxisOrientationType crossAxis = aAxisTracker.GetCrossAxis();
   // We stretch IFF we are align-self:stretch, have no auto margins in
   // cross axis, and have cross-axis size property == "auto". If any of those
   // conditions don't hold up, we won't stretch.
   if (mAlignSelf != NS_STYLE_ALIGN_STRETCH ||
       GetNumAutoMarginsInAxis(crossAxis) != 0 ||
-      eStyleUnit_Auto != aAxisTracker.ComputedCrossSize(mFrame).GetUnit()) {
+      !IsCrossSizeAuto()) {
     return;
   }
 
   // If we've already been stretched, we can bail out early, too.
   // No need to redo the calculation.
   if (mIsStretched) {
     return;
   }