‘idx’ is where _new_ data should be pushed into the buffer. Thus it is
perfectly valid for it to be equal to ‘size’ - it just means we need
to allocate more space before pushing data to it.
When enabled, the mouse cursor is hidden when the user types in the
terminal. It is un-hidden when the user moves the mouse, or when the
window loses keyboard focus.
This allows us to safely call extract_finish() when extract_one()
failed, and we'll get the expected result; false, indicating
extract_finish() failed.
