Interactive popups close when entering nested popups

1. Call `:LeanTermGoal` twice, this opens a floating window and moves the cursor into it.
2. Open a nested popup and move the cursor into the nested popup. (requires Lean 4)
3. The floating window disappears and the nested popup moves to the top-left corner.

This is probably due to this change in neovim 0.6: https://github.com/neovim/neovim/pull/16557 I'm not sure what the best solution is, but we might need to roll our own floating windows.