This allows us to return early from a `write`, which we need in cases
where we want to perform an operation inbetween `read` and `write` -
namely, callback for ButtonRequest should technically be invoked after
returning ButtonAck but before waiting for device's response.
Of course that doesn't really work. The callback will actually be
invoked _before_ ButtonAck, so there's still the condition that it must
return immediately or the device gets stuck with a black screen.
But doing this allows us to write code *as if* it worked, which lets the
other transports run free and wild, by which I mean, do the Right Thing
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