I think what you want is for the mini-window to shrink to zero lines. The mini-window's minimal height is currently fixed at one line: it can't be made (permanently) taller or smaller than that.

Since I'm currently trying to understand the mini-window resize code anyway, here's a patch that does that. It's far from complete (in particular, mouseover on the mode line is broken) and in no way fit for inclusion, so do be careful if you decide to test it.

However, having tried it, it's really annoying! IMHO, it would be much better to have the mini-window in a separate frame and auto-raise that when it's needed, possibly obstructing the modeline.

On Thu, Aug 27, 2015 at 3:41 PM, Q <godblessfq@gmail.com> wrote:

Hello everyone,

Maximize the editing area can be very desirable on netbooks and
phones. In most of the times the echo area is blank, and the
information on the modeline is glanced only once in a while. I am
wondering if the modeline info can be displayed in the echo area
whenever there is no message? Or if a hook can be added for
detecting echo area changes?

There is a stackexchange question on this
http://emacs.stackexchange.com/questions/7563/make-use-of-an-empty-echo-area-to-display-information

Thank you for your consideration!
Q