Edit

This feature is implemented. It is directly available in newly created Flutter project since Flutter 3.19.
For older project, the following section should be added to the Android manifest file:

 <!--  Required to query activities that can process text, see:
       https://developer.android.com/training/package-visibility and
       https://developer.android.com/reference/android/content/Intent#ACTION_PROCESS_TEXT.

       In particular, this is used by the Flutter engine in io.flutter.plugin.text.ProcessTextPlugin.  -->
<queries>
  <intent>
    <action android:name="android.intent.action.PROCESS_TEXT"/>
    <data android:mimeType="text/plain"/>
  </intent>
</queries>

Use case

On Android, it's possible to write an app that adds a custom button to the system-wide text selection toolbar. For example, here the Android app AnkiDroid has added the "Anki Card" button to my text selection toolbar, and it appears in any app, such as the Google app here.

Proposal

Flutter should respect these added buttons, so that in any Flutter app running on Android, the buttons that the user has added to their device show up in the default text selection toolbar just like in any native app.

This is the Android part of #107603 which is broad (all plaforms).
Creating this issue to make it easier to follow.

Implementation steps