Why Red rectangle around the android UI

[conteczxy]

Hi,

I found sometimes, there is a red rectangle around the android UI, like this,

It shows suddently, and then dispear very quickly. what is it used for?

Why it show? My platform is android 4.4.

Thanks,

[rickchu]

Its because Android compilation as engineer mode; the red rectangle is displayed for debug usage.
you can change TARGET_BUILD_VARIANT=user (or userdebug) to make red rectangle disappear.