Remove -khtml-user-select. It's not supported by KHTML anyways and their marketshare is incredibly minor.

select2.css

@@ -45,7 +45,6 @@ Version: @@ver@@ Timestamp: @@timestamp@@
 
     -webkit-touch-callout: none;
       -webkit-user-select: none;
-       -khtml-user-select: none;
          -moz-user-select: none;
           -ms-user-select: none;
               user-select: none;
@@ -343,7 +342,6 @@ Version: @@ver@@ Timestamp: @@timestamp@@
 
     -webkit-touch-callout: none;
       -webkit-user-select: none;
-       -khtml-user-select: none;
          -moz-user-select: none;
           -ms-user-select: none;
               user-select: none;
@@ -510,7 +508,6 @@ disabled look for disabled choices in the results dropdown
 
     -webkit-touch-callout: none;
       -webkit-user-select: none;
-       -khtml-user-select: none;
          -moz-user-select: none;
           -ms-user-select: none;
               user-select: none;