https://bitbucket.org/rsc/drawterm/issue/7/drawterm-emulation-of-2-mouse-button-does#comment-16264332
--- gui-x11/x11.c     Sun Jan 02 18:33:44 2011 -0500
+++ gui-x11/x11.c     Sat May 05 01:00:27 2012 -0700
@@ -1340,8 +1340,12 @@
                ms.buttons |= 1;
        if(s & Button2Mask)
                ms.buttons |= 2;
-       if(s & Button3Mask)
-               ms.buttons |= 4;
+       if(s & Button3Mask){
+              if (s & ShiftMask)
+                    ms.buttons |= 2;
+              else
+                    ms.buttons |= 4;
+       }
        if(s & Button4Mask)
                ms.buttons |= 8;
        if(s & Button5Mask)
