Issue #1986 - Part 2: Add IsItemInlineAxisMainAxis() and rework some nsFrame code.

It turns out part 10 was actually needed, but Bug 1449838 had a significantly
better version of what that code does that was easy enough to backport to what
we have. As far as I can tell, this passes the relevant reftests now.

3 files changed, 58 insertions, 7 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,
diff --git a/layout/generic/nsFlexContainerFrame.h b/layout/generic/nsFlexContainerFrame.h
index 42ccdfeb85..e349709876 100644
--- a/layout/generic/nsFlexContainerFrame.h
+++ b/layout/generic/nsFlexContainerFrame.h
@@ -133,6 +133,21 @@ public:
                                     uint32_t* aNumPackingSpacesRemaining,
                                     nscoord* aPackingSpaceRemaining);
 
+  /**
+   * Given a frame for a flex item, this method returns true IFF that flex
+   * item's inline axis is the same as (i.e. not orthogonal to) its flex
+   * container's main axis.
+   *
+   * (This method is only intended to be used from external
+   * callers. Inside of flex reflow code, FlexItem::IsInlineAxisMainAxis() is
+   * equivalent & more optimal.)
+   *
+   * @param aFrame a flex item (must return true from IsFlexItem)
+   * @return true iff aFrame's inline axis is the same as (i.e. not orthogonal
+   *              to) its flex container's main axis. Otherwise, false.
+   */
+  static bool IsItemInlineAxisMainAxis(nsIFrame* aFrame);
+
 protected:
   // Protected constructor & destructor
   explicit nsFlexContainerFrame(nsStyleContext* aContext)
diff --git a/layout/generic/nsFrame.cpp b/layout/generic/nsFrame.cpp
index 43ad970890..9bc4941e0b 100644
--- a/layout/generic/nsFrame.cpp
+++ b/layout/generic/nsFrame.cpp
@@ -39,6 +39,7 @@
 #include "mozilla/Logging.h"
 #include "mozilla/Sprintf.h"
 #include "nsFrameManager.h"
+#include "nsFlexContainerFrame.h"
 #include "nsLayoutUtils.h"
 #include "LayoutLogging.h"
 #include "mozilla/RestyleManager.h"
@@ -4752,12 +4753,11 @@ nsFrame::ComputeSize(nsRenderingContext* aRenderingContext,
   }
   bool isFlexItem = (parentFrameType == nsGkAtoms::flexContainerFrame &&
                      !(GetStateBits() & NS_FRAME_OUT_OF_FLOW));
+
   bool isInlineFlexItem = false;
   if (isFlexItem) {
-    uint32_t flexDirection = GetParent()->StylePosition()->mFlexDirection;
     isInlineFlexItem =
-      flexDirection == NS_STYLE_FLEX_DIRECTION_ROW ||
-      flexDirection == NS_STYLE_FLEX_DIRECTION_ROW_REVERSE;
+      nsFlexContainerFrame::IsItemInlineAxisMainAxis(this);
 
     const nsStyleCoord* flexBasis = &(stylePos->mFlexBasis);
     SetCoordToFlexBasis(isInlineFlexItem, false, flexBasis,
@@ -4973,11 +4973,8 @@ nsFrame::ComputeSizeWithIntrinsicDimensions(nsRenderingContext*  aRenderingConte
   // from our style struct. (Otherwise, we'll be using an irrelevant value in
   // the aspect-ratio calculations below.)
   if (isFlexItem) {
-    uint32_t flexDirection =
-      GetParent()->StylePosition()->mFlexDirection;
     isInlineFlexItem =
-      flexDirection == NS_STYLE_FLEX_DIRECTION_ROW ||
-      flexDirection == NS_STYLE_FLEX_DIRECTION_ROW_REVERSE;
+      nsFlexContainerFrame::IsItemInlineAxisMainAxis(this);
 
     // If FlexItemMainSizeOverride frame-property is set, then that means the
     // flex container is imposing a main-size on this flex item for it to use