Free select tool

[Ofnuts]

In Gimp 2.10, even after the "connection", you can still change things:

• Depressing the ⌫ key (backspace) undoes the previous step (removes a node, or re-opens the polygon)
  
• Putting the mouse over a node makes a circle appear  and you can drag the node
  

This is valid until you "commit" the selection (double-click, ↵ (enter) or switch to another tool).

For complex stuff using the path tool is more efficient.

You can also work out your selection piecemeal.