Why is this in public GAPI headers?

@anna-khakimova countNonZero() is banned for a reason: this function can take ONLY single-channel Mat.
(See #16657)
You can use cv::norm() here instead

It is banned in tests only.
No idea how this appears in common public header.

This appear because both headers modules/gapi/include/opencv2/gapi/cpu/video.hpp and modules/gapi/test/test_precomp.hpp are included to one modules/gapi/test/cpu/gapi_video_tests_cpu.cpp. So,
#define countNonZero() countNonZero_is_forbidden_in_tests_use_norm_instead() in test_precomp.hpp affects content of cpu/video.hpp file.

Do you really need such details in the future?

Please move this hack to the tests if it is really necessary (or move code implementation into src/, or replace this call)

cv::countNonZero(...) > 0) is slow (and may cause unexpected result overflow which is worse). Replace to

norm(..., NORM_INF) != 0

Ok. Changed.

A lot of code in public headers.

Public headers should define API for Users only.
Implementation must go into "src".

/cc @dmatveev

I think, implementation can be placed in modules/gapi/src/backends/cpu/gcpuvideo.cpp next to the kernel which uses this

Done

Classes in public headers should be properly documented.

Ok. Applied.

@anna-khakimova countNonZero() is banned for a reason: this function can take ONLY single-channel Mat.
(See #16657)
You can use cv::norm() here instead

I think, implementation can be placed in modules/gapi/src/backends/cpu/gcpuvideo.cpp next to the kernel which uses this