x-select-enable-clipboard-manager

x-select-enable-clipboard-manager is a variable defined in `C source code'.
Its value is

t

Documentation:
Whether to enable X clipboard manager support.
If non-nil, then whenever Emacs is killed or an Emacs frame is deleted
while owning the X clipboard, the clipboard contents are saved to the
clipboard manager if one is present.

You can customize this variable.

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