[valentinvoilean]

Use these options to add classes for valid & invalid groups.

sortableOptions = {
        group: {
            name: 'qux',
            put: function (to) {
                return to.el.children.length < 4;
            }
        },
        validGroupClass: 'myValidCLass', // default is "sortable-valid"
        invalidGroupClass: 'myInvalidClass', // default is "'sortable-invalid'
    };

Also, in order to make it work with react, I had to apply this fix too (which was declined before):
#984
If I don't use that fix, the item disappears after is dropped.