allowSorting does not disable sorting on header clicks

[TonyGermaneri]

It does remove the option from the context menu. This is simple to fix.

check the allowSorting bool just before this line

https://github.com/TonyGermaneri/canvas-datagrid/blob/aee9a2a12d43dda54d615e619b90031f37f32527/lib/publicMethods.js#L642

[jswolf19]

As a workaround:

grid.addEventListener('beforesortcolumn', function(e) { e.preventDefault(); });