Automatic now measures whether the system's bottom buttons are actually
over the keyboard, instead of guessing from the insets.

The insets describe the screen, not this window. Two phones can report
the same navigation bar while one draws the keyboard underneath it and
the other has already placed the keyboard above it - and the second was
losing a strip of keyboard for furniture that was not on top of it.

The keyboard now compares its own bottom edge against the top of the
navigation bar and reserves only what is genuinely over it. Nothing over
it, nothing reserved.
