fix edge-case where user can right-click on a newly vis'd canvas before it has finished mapping

pd/nw/pd_canvas.html

@@ -21,7 +21,8 @@
     <svg xmlns="http://www.w3.org/2000/svg"
          version="1.1"
          id="patchsvg"
-         class="noselect">
+         class="noselect"
+         viewBox="0 0 0 0">
     </svg>
     <div id="canvas_find" style="display:none;">
       <label><input type="text"