Yam Display for macOS High Sierra.

Extend Display by AirPlay® for macOS High Sierra (10.13.4 ~ 10.13.6)
Yam Display uses AirPlay® to extend display for macOS High Sierra (10.13.4 ~ 10.13.6) due to an issue. There can be issues for older Macs (released before 2011) that do not support AirPlay mirroring. Resolutions are limited to horizontal ones. The issue was resolved in macOS Mojave in which full screen resolutions including portrait mode were restored.