As of 6/4/21 with OS10.0.0-4195 and RokuCast 1.9.6 and AirPlay2 1.14.104, screen mirroring is now working with Composite output on the 3910 for both Miracast & AirPlay2 based connections.
Previously (all previous firmware/app versions) composite output was excluded/non-functional when mirroring (RokuCast/AirPlay2 would close to home upon connecting):
https://support.roku.com/article/208754928
Does my Roku device support screen mirroring?
Most current generation Roku streaming players and Roku TVs support screen mirroring with notable exceptions listed below. To find the model of your Roku device, go to Settings > System > About.
- Roku Express: Screen mirroring is supported on models 3930 and 3900, but not on model 3700.
- Roku Express+: Screen mirroring is supported on models 3931 and 3910, but not on model 3710. On model 3910, screen mirroring is only supported on the HDMI output, and not on the composite, or A/V output.
As of 6/4/21 this information is no longer accurate/true, and mirroring (Miracast and AirPlay2) now works in composite output mode.
