Horizontal scroll denied in source code of sub_row_grid excell.
dhtmlxgrid_excell_sub.js, line 205
td._sub_grid.objBox.style.overflow=“hidden”;
can be changed to
td._sub_grid.objBox.style.overflowY=“hidden”;
or you can add next line of code
grid.attachEvent(“onSubGridLoaded”,function(sub){
sub.objBox.style.overflowX=“auto”;
return true;
})