shinyTable | An R package that integrates Handsontable with Shiny | Grid library

This should get you started. As per my comment, you should use rhandsontable. This package uses the same underlying JS library, handsontable.JS, but it is well supported and it is on Cran (disclaimer: I'm a minor contributor to this package).

The working example below is based on rhandsontable. For simplicity I've only implemented the change of the number of rows.

Please take into account that I haven't implemented any type of caching mechanism, either to a global variable, or to a reactive variable, as it wasn't necessary, but it can easily be added.

This is the only example that I know of a library working in shiny where there is an output$something linked to an input$something.

In this case the input$tbl in the code refer to the table, but to be converted to a data frame it needs to be transformed by the convenience function hot_to_r (handsontable to R).

I am sure you are already familiar with this: you use hot_to_r(input$tbl) to check if the user has changed anything in the displayed table (assuming it is not read-only). shinyTable has a much more complicated mechanism, but it is prone to races.