Having nothing better to do, I just pulled apart a junk keyboard.
Inside, it had the regular capacitive sensors separated by Mylar
sheet. In addition, there was a large piece of metal, almost as big as
the keyboard area.

This metal is very soft, and is permeable. it's definitely not
Aluminum. It was just sandwiched inside, with nothing screwed to it,
so it deosn't seem to be a strengthening device. I'm wondering if it
is a magnetic shield. Is there a way to tell whether it is MuMetal or
just ordinary soft iron? Would one expect a big chunk of MuMetal
inside a capacitive keyboard?

Neil

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