When many tasks exist in a list, selecting a task at mid of the list and complete it is difficult, because "Complete" button is too far.
I think buttons of "Complete" and "Postpone" and pop-up of "More Actions..." should be right pane showing description of task and note, and be floating from list scrolling.

And I think original "Complete" button should be inactive when no tasks are selected.
It button must mean "to complete selected tasks".
Original "Postpone" button, too.