don't assume button information is still present in mousemove event

Author: marijnh

lib/codemirror.js

@@ -221,7 +221,6 @@ var CodeMirror = (function() {
       var move = connect(document, "mousemove", operation(function(e) {
         clearTimeout(going);
         e.stop();
-        if (!e.button()) {end(); return;}
         extend(e);
       }), true);
       var up = connect(document, "mouseup", operation(function(e) {