select-active-regions

select-active-regions is a variable defined in `C source code'.
Its value is

t

Documentation:
If non-nil, an active region automatically sets the primary selection.
If the value is `only', only temporarily active regions (usually made
by mouse-dragging or shift-selection) set the window selection.

This takes effect only when Transient Mark mode is enabled.

You can customize this variable.

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