new gnumeric
set up validation for A1 to only allow whole numbers between 0 and 9 with the action to never allow invalid input
attempt to enter 2.5
validation intercedes and correctly prohibits entering of that number. Discard.
Note that entering '2.5" is not on the redo menu.
selecting that item from the redo menu enters 2.5 in the cell without validation.

This item should never appear on the redo menu! It is not properly an undone action.

The cause for this is that wbcg_edit_finish calls command-undo if validation fails since it already committed the changes using cmd_set_text or cmd_set_array_text. I think this should really be done by setting up the correct GOUndos and only invoking the cmd_... (probably cmd_generic) when the action has been validated!

In fact I think that wbcg_edit_finish should be devided into a wbcg_edit_finish_construct that creates an undo and a redo GOUndo and a wrapper that commits those undos.

Since validation could depend on external data one could make a case that validation should also happen on 'redo' but that is arguable.