diff --git a/mozilla/layout/generic/nsIntervalSet.cpp b/mozilla/layout/generic/nsIntervalSet.cpp
index ce6a9aca31f..f103963523f 100644
--- a/mozilla/layout/generic/nsIntervalSet.cpp
+++ b/mozilla/layout/generic/nsIntervalSet.cpp
@@ -88,6 +88,7 @@ void nsIntervalSet::IncludeInterval(coord_type aBegin, coord_type aEnd)
 
     Interval *subsumed = newInterval->mNext;
     while (subsumed && subsumed->mBegin <= aEnd) {
+        newInterval->mBegin = PR_MIN(newInterval->mBegin, subsumed->mBegin);
         newInterval->mEnd = PR_MAX(newInterval->mEnd, subsumed->mEnd);
         newInterval->mNext = subsumed->mNext;
         FreeInterval(subsumed);