Make the selection able to exist independent of the cursor position, so

that it is not affected by scrolling. If MouseDragEnd1Pane is bound to
the new "stop-selection" command:

    bind -Tcopy-mode MouseDragEnd1Pane stop-selection

A selection made with the mouse will stay as it is after button 1 is
released. (It also works bound to a key.)

From Artem Fokin.

tmux.1

@@ -1072,6 +1072,7 @@ The following commands are supported in copy mode:
 .It Li "search-reverse" Ta "N" Ta "N"
 .It Li "select-line" Ta "V" Ta ""
 .It Li "start-of-line" Ta "0" Ta "C-a"
+.It Li "stop-selection" Ta "" Ta ""
 .It Li "top-line" Ta "H" Ta "M-R"
 .El
 .Pp