To extend a selected region that was selected with the mouse,
you hold the shift key while clicking.

See also terminal shortcuts.

Key	Action	Notes
Shift+Ctrl+t	new tab
Shift+Ctrl+w	close tab
Shift+Ctrl+n	new window
Ctrl+{PgUp,PgDown}	change tabs
Shift+Insert	paste from selection buffer, or if that's empty the clipboard
Ctrl+Insert	copy selection to clipboard
Shift+Ctrl+v	paste from clipboard
Ctrl+Click	Open URL under mouse cursor