A new option type next to column-mapping: pick a subset of columns and order them —
e.g. group-by or order-by columns. Same two-pane layout as the mapping, but the left pane rows carry
tickboxes, the gutter stays empty (no edges, no dots), and the right pane lists the ticked columns
in a user-controlled order (drag to rearrange, same grip as the mapping's output reorder).
Try it: tick/untick columns on the left — they appear/disappear on the right (new ticks append at the end).
Drag a right-hand row by its grip (or its column icon) to reorder. Flip the author settings below:
source switches the left pane between input and output columns; sort order adds an
ASC/DESC toggle to every selected column, in the slot where the mapping shows the expression.