use prevent default in drag

contrib/js/backlink.js

@@ -332,6 +332,7 @@ function init(prefix) {
   e.addEventListener("dragstart", function(ev) {
     mouseDownX = ev.clientX;
     mouseDownY = ev.clientY;
+    ev.preventDefault();
   }, false);
   
   e.addEventListener("dragend", function(ev) {
@@ -342,6 +343,7 @@ function init(prefix) {
     rpl.moveTo(x, y);
     originalX = x;
     originalY = y;
+    ev.preventDefault();
   }, false);
 
   // add replyto post handlers