1 files changed, 39 insertions, 0 deletions
diff --git a/layout/generic/nsFlexContainerFrame.cpp b/layout/generic/nsFlexContainerFrame.cpp
index 02d15b5907..b4907a4862 100644
--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -1990,6 +1990,10 @@ FlexItem::FlexItem(ReflowInput& aFlexItemReflowInput,
              "placeholder frames should not be treated as flex items");
   MOZ_ASSERT(!(mFrame->GetStateBits() & NS_FRAME_OUT_OF_FLOW),
              "out-of-flow frames should not be treated as flex items");
+  MOZ_ASSERT(mIsInlineAxisMainAxis ==
+             nsFlexContainerFrame::IsItemInlineAxisMainAxis(mFrame),
+             "public API should be consistent with internal state (about "
+             "whether flex item's inline axis is flex container's main axis)");
 
   const ReflowInput* containerRS = aFlexItemReflowInput.mParentReflowInput;
   if (IsLegacyBox(containerRS->mFrame)) {
@@ -4421,6 +4425,41 @@ nsFlexContainerFrame::CalculatePackingSpace(uint32_t aNumThingsToPack,
   *aPackingSpaceRemaining -= totalEdgePackingSpace;
 }
 
+/* static */
+bool
+nsFlexContainerFrame::IsItemInlineAxisMainAxis(nsIFrame* aFrame)
+{
+  MOZ_ASSERT(aFrame && aFrame->IsFlexItem(), "expecting arg to be a flex item");
+  const WritingMode flexItemWM = aFrame->GetWritingMode();
+  const nsIFrame* flexContainer = aFrame->GetParent();
+
+  if (IsLegacyBox(flexContainer)) {
+    // For legacy boxes, the main axis is determined by "box-orient", and we can
+    // just directly check if that's vertical, and compare that to whether the
+    // item's WM is also vertical:
+    bool boxOrientIsVertical =
+      (flexContainer->StyleXUL()->mBoxOrient == StyleBoxOrient::Vertical);
+    return flexItemWM.IsVertical() == boxOrientIsVertical;
+  }
+
+  // For modern CSS flexbox, we get our return value by asking two questions
+  // and comparing their answers.
+  // Question 1: does aFrame have the same inline axis as its flex container?
+  bool itemInlineAxisIsParallelToParent =
+    !flexItemWM.IsOrthogonalTo(flexContainer->GetWritingMode());
+
+  // Question 2: is aFrame's flex container row-oriented? (This tells us
+  // whether the flex container's main axis is its inline axis.)
+  auto flexDirection = flexContainer->StylePosition()->mFlexDirection;
+  bool flexContainerIsRowOriented =
+    flexDirection == NS_STYLE_FLEX_DIRECTION_ROW ||
+    flexDirection == NS_STYLE_FLEX_DIRECTION_ROW_REVERSE;
+
+  // aFrame's inline axis is its flex container's main axis IFF the above
+  // questions have the same answer.
+  return flexContainerIsRowOriented == itemInlineAxisIsParallelToParent;
+}
+
 void
 nsFlexContainerFrame::DoFlexLayout(nsPresContext*           aPresContext,
                                    ReflowOutput&     aDesiredSize,