ionic2 flex layout action button will disappear both on phone and browsers

[davyzhang]

video to of this problem
https://dl.dropboxusercontent.com/u/84845057/flex.mov

code of this app
https://dl.dropboxusercontent.com/u/84845057/sp.zip

I found its a bit about the z-index of button

ionic serve -t android will reproduce this problem

but

ionic serve -t ios works fine