mouse-drag-copy-region

mouse-drag-copy-region is a variable defined in `mouse.el'.
Its value is

nil

Documentation:
If non-nil, copy to kill-ring upon mouse adjustments of the region.

This affects `mouse-save-then-kill' () in
addition to mouse drags.

You can customize this variable.

This variable was introduced, or its default value was changed, in version 24.1 of Emacs.