NewPlanner
Planner: Task Details Side Pane experience
Planner users can now view and edit task details in a side pane instead of modal dialogs, allowing them to maintain context of their Board, Grid, or My Tasks view while making updates.
Microsoft's description
The Task Details Side Pane lets you view and edit task details in a side pane while keeping your Board, Grid, or My Tasks view visible. Update tasks without losing context or switching back and forth from modal dialogs.