One of the planners noticed that they used to have the ability to right click, select order. Then, they could hold down the control key and deselect certain tasks, if they did not want to move them on the planning board with the other tasks in the order.
Planners can no longer deselect tasks manually on the board.
Solution
This is a local environment issue. Please check your display settings and ask your IT department to check the server's display adapters are up to date.
A work around is to use the Task Planner from the left hand pane to select/deselect tasks.