In sandbox / practice mode, there is no undo button

Pressing u does not undo. It seems it hasn't been implemented yet.

+1 I want to recommend this as a learning tool but without undo, the experience feels fairly clunky. I'm already an avid vim user though, so the experience might be different if you're just starting out.

There is undo only for the latest command, it seems.

I understand that undo would be helpful, but I won't be using time to develop OpenVim anymore -- other than fixing critical bugs.

OpenVim could be much more useful tool to learn Vim if I decided to put energy on to make it full-blown learning tool, but I rather not, and I think OpenVim is at a good local optimum: to provide a quick taste of Vim.

Thanks for the feedback, though.