> > Is it possible to test the Fn key?
> > I have created a custom layout (based one of the
> > supplied ones), all the keys work except the Fn key.

The Fn key is being intercepted by the hardware or BIOS. So there is no way to test it directly.

If you need to test it, I suggest you make a dummy key on the image and assign it to one of the numeric keypad keys (e.g. the number 7 on the numeric keypad). To get this dummy key to highlight, the tester will need to use the Fn key and thus indirectly will test the key.

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David
PassMark