Fix #1259 for \mathop and \operatorname as well.

Author: dpvc

unpacked/jax/input/TeX/jax.js

@@ -1453,12 +1453,12 @@
 
     Overset: function (name) {
       var top = this.ParseArg(name), base = this.ParseArg(name);
-      if (base.isa(MML.mo)) base.movablelimits = false;
+      if (base.movablelimits) base.movablelimits = false;
       this.Push(MML.mover(base,top));
     },
     Underset: function (name) {
       var bot = this.ParseArg(name), base = this.ParseArg(name);
-      if (base.isa(MML.mo)) base.movablelimits = false;
+      if (base.movablelimits) base.movablelimits = false;
       this.Push(MML.munder(base,bot));
     },