[noggin]

i had read about on reddit  a google survey about "auto frequency switching" , lot of people don't like "black screen" so google doesn't have developed this function.
So maybe now with "non-seamless" (no black screen ) , google have update API Display.Mode for auto frequency switching.... we hope .

I think app-controlled switching - as on the Apple TV - is still the best route (and it's easier to implement with DRM as you don't need any access to the now-playing video stream to detect refresh rate - it's separate metadata that the app passes to the OS). 

Apple TV should be a "global" auto-switching , the setting is for system player. Apps don't have control ,right?

Not right.  You are misunderstanding how tvOS handles refresh rate switching.

The global setting that enables frame rate matching is a setting that enables apps to control the refresh rate, it doesn't enable OS-detection of the refresh rate of video being played.  That setting isn't 'Enable Auto Refresh Rate' it's 'Enable App Controlled Refresh Rate' in reality. 

When the Frame Rate setting is enabled in Settings, it allows apps to signal to the OS that they are playing video at a certain refresh rate, and this triggers the OS to change the Apple TV output refresh rate.  It isn't automatic - it's controlled by the code in the app.  

https://developer.apple.com/videos/play/tech-talks/503/.  (NB 1000/1001 mode support is now there I believe)

if you watch that - apps have to enable the match-frame rate switching within them AND HLS playlists also have to have frame rate information in them, and there is guidance about how to code apps to use frame rate switches intelligently.

For instance - if I am in Netflix, and have 2160p50 as my standard refresh rate, and then play a 2160p23.976 show, the Netflix app tells tvOS that the video being played should change frame rate and the Apple TV changes output refresh rate when it starts playing.   This 'feels' automatic - but it isn't, it requires the app to support it.

If, however, the Netflix app autoplays a 23.976p trailer for the same show in the UI, the refresh rate stays at 2160p50 because the Netflix app has told tvOS not to change frame rate, and so you don't get a black-screen as the TV re-syncs.  (This is what used to happen with Roku devices that DID have automatic refresh rate switching - and is why Roku disabled it entirely in Netflix - and why my Roku is now never used...)

You see the ATV 4K refresh rate switching is NOT entirely automatic, it requires apps to be coded to use it (though it may be easier to code than Android TV's).  Effectively it's the same as Android TV's - except for some reason app developers use it in tvOS but don't in Android TV?