Hello, first of all, thank you for your great app, I have an Nvidia Shield tv 2015 and it works very well.
I've something to share, I tried to disable secure flag system wide, I managed to get there by unlocking the bootloader, downloading the boot.img fron the nvidia site, patching it with another phone with magisk installed (maybe there other ways but this seems to be the only one working for nvidia shield), flashed it to the shield through fastboot, installed magisk module for xposed framework api 26 (for oreo 8.0, other version will use other apis), xposed installer apk for magisk, and disable secure flag xposed module.
Now I'm allowed to take screenshot everywhere, and your screen grabber can grab at least the netflix menu (before it couldn't) but sadly the screenshot while playing netflix or amazon prime video are still black, the great think is that i can grab from the modded netflix apk from xda (https://forum.xda-developers.c…ahd-dolby-5-1-l1-t3535349)
I think it's worth some more investigation, I'm pretty sure that once disabled the secure flag the problem still remaining is widevine L1 since the L3 version works, but I think I can't do much more with my skill level