In order for apps like Messenger to show up on top of other apps, the "floating windows" feature must be enabled on the Oppo smartphone so that apps can show up on top of other apps and provide quick access to its features.

Android 10 (ColorOS 7.0)
Step 1:  Open the  Settings
Step 2:  Tap on  App management
Step 3:  Tap on  Display over other apps
Step 4:  Enable or Disable desired  App
  1. Open the Settings
  2. Tap on App management
  3. Tap on Display over other apps
  4. Enable or Disable desired App

Oppo Instructions
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