--- a/layout/base/nsChangeHint.h
+++ b/layout/base/nsChangeHint.h
@@ -384,16 +384,49 @@ static_assert(!(nsChangeHint_Hints_Alway
   nsChangeHint(nsChangeHint_RepaintFrame | nsChangeHint_SyncFrameView | \
                nsChangeHint_SchedulePaint)
 #define nsChangeHint_AllReflowHints                     \
   nsChangeHint(nsChangeHint_NeedReflow |                \
                nsChangeHint_ReflowChangesSizeOrPosition|\
                nsChangeHint_ClearAncestorIntrinsics |   \
                nsChangeHint_ClearDescendantIntrinsics | \
                nsChangeHint_NeedDirtyReflow)
+
+// Below are the change hints that we send for ISize & BSize changes.
+// Each is similar to nsChangeHint_AllReflowHints with a few changes.
+
+// * For an ISize change, we send nsChangeHint_AllReflowHints, with two bits
+// excluded: nsChangeHint_ClearDescendantIntrinsics (because an ancestor's
+// inline-size change can't affect descendant intrinsic sizes), and
+// nsChangeHint_NeedDirtyReflow (because ISize changes don't need to *force*
+// all descendants to reflow).
+#define nsChangeHint_ReflowHintsForISizeChange            \
+  nsChangeHint(nsChangeHint_AllReflowHints &              \
+               ~(nsChangeHint_ClearDescendantIntrinsics | \
+                 nsChangeHint_NeedDirtyReflow))
+
+// * For a BSize change, we send almost the same hints as for ISize changes,
+// with one extra: nsChangeHint_UpdateComputedBSize.  We need this hint because
+// BSize changes CAN affect descendant intrinsic sizes, due to replaced
+// elements with percentage BSizes in descendants which also have percentage
+// BSizes. nsChangeHint_UpdateComputedBSize clears intrinsic sizes for frames
+// that have such replaced elements. (We could instead send
+// nsChangeHint_ClearDescendantIntrinsics, but that's broader than we need.)
+//
+// NOTE: You might think that BSize changes could exclude
+// nsChangeHint_ClearAncestorIntrinsics (which is inline-axis specific), but we
+// do need to send it, to clear cached results from CSS Flex measuring reflows.
+//
+// XXXdholbert The next patch rewrites this hint to be similar to the ISize one.
+#define nsChangeHint_ReflowHintsForBSizeChange            \
+  nsChangeHint(nsChangeHint_NeedReflow |                  \
+               nsChangeHint_UpdateComputedBSize |         \
+               nsChangeHint_ReflowChangesSizeOrPosition | \
+               nsChangeHint_ClearAncestorIntrinsics)
+
 #define NS_STYLE_HINT_REFLOW \
   nsChangeHint(NS_STYLE_HINT_VISUAL | nsChangeHint_AllReflowHints)
 
 #define nsChangeHint_Hints_CanIgnoreIfNotVisible   \
   nsChangeHint(NS_STYLE_HINT_VISUAL |              \
                nsChangeHint_NeutralChange |        \
                nsChangeHint_UpdateOpacityLayer |   \
                nsChangeHint_UpdateTransformLayer | \

--- a/layout/style/nsStyleStruct.cpp
+++ b/layout/style/nsStyleStruct.cpp
@@ -1638,38 +1638,21 @@ nsStylePosition::CalcDifference(const ns
   // nsStyleContext::CalcStyleDifference, which can lead to incorrect
   // style data.
   // It doesn't matter whether we're looking at the old or new
   // visibility struct, since a change between vertical and horizontal
   // writing-mode will cause a reframe, and it's easier to pass the old.
   if (aOldStyleVisibility) {
     bool isVertical = WritingMode(aOldStyleVisibility).IsVertical();
     if (isVertical ? widthChanged : heightChanged) {
-      // Block-size changes can affect descendant intrinsic sizes due to
-      // replaced elements with percentage bsizes in descendants which
-      // also have percentage bsizes. This is handled via
-      // nsChangeHint_UpdateComputedBSize which clears intrinsic sizes
-      // for frames that have such replaced elements.
-      //
-      // We need to use nsChangeHint_ClearAncestorIntrinsics for
-      // block-size changes so we clear results of cached CSS Flex
-      // measuring reflows.
-      hint |= nsChangeHint_NeedReflow |
-              nsChangeHint_UpdateComputedBSize |
-              nsChangeHint_ReflowChangesSizeOrPosition |
-              nsChangeHint_ClearAncestorIntrinsics;
+      hint |= nsChangeHint_ReflowHintsForBSizeChange;
     }
 
     if (isVertical ? heightChanged : widthChanged) {
-      // None of our inline-size differences can affect descendant
-      // intrinsic sizes and none of them need to force children to
-      // reflow.
-      hint |= nsChangeHint_AllReflowHints &
-              ~(nsChangeHint_ClearDescendantIntrinsics |
-                nsChangeHint_NeedDirtyReflow);
+      hint |= nsChangeHint_ReflowHintsForISizeChange;
     }
   } else {
     if (widthChanged || heightChanged) {
       hint |= nsChangeHint_NeutralChange;
     }
   }
 
   // If any of the offsets have changed, then return the respective hints