how to focus different `regions`(Ctrl-a S) via mouse-click in `screen`?

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http://unix.stackexchange.com – When I start vim in a screen session, and activate the mouse: :set mouse=a the mouse remains disabled. I cannot switch tabs via mouse click. How can I configure screen so that mouse support in vim works? Edit: I guess the version of my screen is too old. When I upgrade it, mouse works fine in vim. I still want to know how to focus different regions(Ctrl-a S) via mouse click. (HowTos)