diff --git a/layout/generic/nsFlexContainerFrame.cpp b/layout/generic/nsFlexContainerFrame.cpp
index 07a48dd36165..0904be796719 100644
--- a/layout/generic/nsFlexContainerFrame.cpp
+++ b/layout/generic/nsFlexContainerFrame.cpp
@@ -3492,13 +3492,17 @@ MainAxisPositionTracker::MainAxisPositionTracker(
mPackingSpaceRemaining -= aLine->SumOfGaps();
// If packing space is negative or we only have one item, 'space-between'
- // falls back to 'safe flex-start', and 'space-around' & 'space-evenly' fall
- // back to 'safe center'. In those cases, it's simplest to just pretend we
- // have a different 'justify-content' value and share code.
+ // falls back to 'safe flex-start'[1], and 'space-around' & 'space-evenly'
+ // fall back to 'safe center'. In those cases, it's simplest to just pretend
+ // we have a different 'justify-content' value and share code.
// https://drafts.csswg.org/css-align-3/#distribution-values
if (mPackingSpaceRemaining < 0 || aLine->NumItems() == 1) {
if (mJustifyContent.primary == StyleAlignFlags::SPACE_BETWEEN) {
- justifyContentFlags = StyleAlignFlags::SAFE;
+ // [1] XXXdholbert Per spec, we should do...
+ // justifyContentFlags = StyleAlignFlags::SAFE;
+ // ...here, but for now let's not do that since it causes overflow to be
+ // inadvertently clipped in situations with a reversed main axis!
+ // See https://github.com/w3c/csswg-drafts/issues/11937
mJustifyContent.primary = StyleAlignFlags::FLEX_START;
} else if (mJustifyContent.primary == StyleAlignFlags::SPACE_AROUND ||
mJustifyContent.primary == StyleAlignFlags::SPACE_EVENLY) {
diff --git a/testing/web-platform/tests/css/css-flexbox/justify-content_space-between-003.tentative.html b/testing/web-platform/tests/css/css-flexbox/justify-content_space-between-003.tentative.html
new file mode 100644
index 000000000000..dfe14a868355
--- /dev/null
+++ b/testing/web-platform/tests/css/css-flexbox/justify-content_space-between-003.tentative.html
@@ -0,0 +1,33 @@
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