Current behavior:
1. When mirror mode is on, all displays are showing the same content in the source display’s current resolution. For now, we are using the internal display’s current resolution or the first external display if internal display does not exist.

2. Kiosk app wants to mirror a subset of displays, currently we decided to mirror in the highest native resolution of all displays in the subset. (native resolution is the highest resolution one display has.)

Using different source in the above two use cases does not make sense to users, so we should have a simpler behavior here and always use the display with highest native resolution as source.

In mirror mode, the resolution slider in the display settings allows user to adjust the source display’s resolution. Using highest-native-resolution display as source also gives user option to mirror in 4k if we have an external 4k display.