mouse-drag-region

mouse-drag-region is an interactive compiled Lisp function in `mouse.el'.

It is bound to <down-mouse-1>.

(mouse-drag-region START-EVENT)

Set the region to the text that the mouse is dragged over.
Highlight the drag area as you move the mouse.
This must be bound to a button-down mouse event.
In Transient Mark mode, the highlighting remains as long as the mark
remains active. Otherwise, it remains until the next input event.