put root z-index above blockly widgets when sim is fullscreen (#10351)

Author: riknoll

theme/common.less

@@ -1374,6 +1374,8 @@ p.ui.font.small {
         fuyllscreensim
 *******************************/
 .fullscreensim {
+    z-index: @aboveEverythingZIndex;
+
     /* Full screen */
     .simPanel {
         position: fixed;