Pixly has a history system, upon which most (if not all) operations are recorded and you can undo and redo them.
From brush strokes to the selection, all operations will be recorded.
The undo and redo buttons, represented by green arrows are located in the bottom-left corner
The arrow pointing left is the Undo, likewise, the one pointing right is the Redo.
A lit button represents that there's operations to be undone or redone, a unlit button shows that there's no operation to be undone or redone.
There are plans to have a History list, where you can see every action you did, and quickly go back or forth in time.