For Samsung device: To prevent the app stoped by itself:
System Settings > Device Maintenance > Battery > Unmonitored apps > Add apps > checked Partial Screen
For Oppo device: To prevent the app stoped by itself:
Security Center > Battery > Enable Smart power-saving mode > Power-saving app control and management > Add apps > checked Partial Screen
For Xiaomi smartphone: need manually grant "Draw over other apps" permission (Go to Setting > Installed apps > Partial Screen > Permission manager > Display pop-up window > "Allow")
To prevent the app turn off when clear RAM: Go to Security Tab > Permission > Auto-start management > Add auto-start apps, checked Partial Screen
For Huawei smartphone: Open Phone Manager apps (or Settings app) > Permission Manager > choose Applications tab > select Partial Screen > enable Draw over other apps