[css-flexbox] Correctly implement contain: size on a flex item

third_party/WebKit/Source/core/layout/LayoutObject.cpp

Index: third_party/WebKit/Source/core/layout/LayoutObject.cpp
diff --git a/third_party/WebKit/Source/core/layout/LayoutObject.cpp b/third_party/WebKit/Source/core/layout/LayoutObject.cpp
index 56fcdcaad14c3c8bea7e21c193d6f9675f918caf..41dba4ef9b60ad6c8700410bd6eb1b2720df4d7f 100644
--- a/third_party/WebKit/Source/core/layout/LayoutObject.cpp
+++ b/third_party/WebKit/Source/core/layout/LayoutObject.cpp
@@ -710,6 +710,11 @@ static inline bool objectIsRelayoutBoundary(const LayoutObject* object)
     if (object->isLayoutScrollbarPart())
         return false;
 
+    // In general we can't relayout a flex item independently of its container; not only is the result
+    // incorrect due to the override size that's set, it also messes with the cached main size on the flexbox.
+    if (object->isBox() && toLayoutBox(object)->isFlexItem())
+        return false;
+
     // Inside multicol it's generally problematic to allow relayout roots. The multicol container
     // itself may be scheduled for relayout as well (due to other changes that may have happened
     // since the previous layout pass), which might affect the column heights, which may affect how