This feature allows you to cancel the sales of aircraft or ground vehicles if needed.
Upon clicking the minus button, a count of sold units will be appended to the unit count. This count responds to both further presses of the minus button, and also to presses of the plus button. No further units will be requested for the next turn until all sold units have been re-bought.
I've tested a bunch of different scenarios with this:
- Selling and rebuying a unit - the budget increases and decreases as expected.
- Selling one unit, buying a unit worth the new player budget, and then trying to rebuy the old unit - the old unit cannot be rebought until the budget has been freed up for it.
- Closing the base window and re-opening it - the sold unit count is retained.
- Ending the turn - the sold unit count is reset back to 0 as expected.
Contributes to Khopa/dcs_liberation#365
