The "EXAMPLE - SHOW A CUSTOM TOOL" at https://docs.telerik.com/kendo-ui/api/javascript/ui/spreadsheet#configuration-toolbar does not seem to work. Using the preview or opening in dojo displays no custom button on the toolbar. It looks like one might be created, but it is invisible unless you hover over the right place, and even then it is just an empty shaded square.
I am trying to add a custom button to the toolbar on spreadsheet and can't seem to get it to work either.