• Component used: AndroidX AppCompat
• Version used: latest, 1.3.0-rc01 (all versions really)
• Devices/Android versions reproduced on: All Android versions, all devices.

Summary

Calling AppCompatActivity.setSupportActionBar() replaces the window callback with a delegate callback that delegates to a previously installed AppCompatWindowCallback instead of delegating to the current window callback, thus breaking developers expectations around window callback delegation.

Root cause

1. Every android.view.Window instance can have a single android.view.Window.Callback set (via Window.setCallback). The correct pattern to support multiple callbacks is to replace the window callback and then delegate to the previously set callback. This correct pattern happens when calling AppCompatActivity.onCreate(), as we can see in AppCompatDelegateImpl.attachToWindow().
2. AppCompatActivity.setSupportActionBar() does not follow that correct pattern. It replaces the callback but then delegates to the previously installed AppCompatWindowCallback instead of the current window callback, as we can see in AppCompatDelegateImpl.setSupportActionBar().

Repro

This bug was originally reported square/curtains#14. After investigating, I determined this is actually an AppCompat bug. I'm attaching the repro project and video demonstrating the bug from that ticket.

Here's an example activity that reproduces the issue:

package com.lb.recordappsample

import android.os.Bundle
import android.util.Log
import android.view.MotionEvent
import android.view.Window
import androidx.appcompat.app.AppCompatActivity

class MainActivity : AppCompatActivity() {

  override fun onCreate(savedInstanceState: Bundle?) {
    super.onCreate(savedInstanceState)
    setContentView(R.layout.activity_main)

    // At this point, window.callback returns AppCompatWindowCallback which delegates
    // to Activity.

    val previousCallback = window.callback

    window.callback = object : Window.Callback by previousCallback {
      override fun dispatchTouchEvent(event: MotionEvent?): Boolean {
        Log.d("MainActivity", "Touch: $event")
        return previousCallback.dispatchTouchEvent(event)
      }
    }

    // At this point, window.callback returns our custom callback which delegates to
    // AppCompatWindowCallback which delegates to Activity.

    setSupportActionBar(findViewById(R.id.toolbar))

    // At this point, window.callback returns ToolbarActionBar.ToolbarCallbackWrapper
    // which delegates to AppCompatWindowCallback which delegates to Activity. Our
    // custom callback is not in the delegate chain any more! That's the bug.
  }
}

The fix is likely that AppCompatDelegateImpl.setSupportActionBar() should pass mWindow.getCallback() to the ToolbarActionBar constructor, instead of mAppCompatWindowCallback.