This looks great, looks like a lot of work has been poured in getting the UX right.

I am the creator of a spreadsheet-like JavaScript product named DataGridXL (https://datagridxl.com). I can tell how much work it is to get all the interactions right!

One thing that confused me is the immediate-drag when clicking on row or column header, I am used that single-click is for selecting, and only drag when the column or row is already selected.

In any case, you guys are on to something here! Best of luck!