Summary:

The Up button is displayed on the action bar for root destinations.

Library:

AndroidX Navigation 2.4.0-alpha01

Description:

When using the AndroidX navigation version 2.4.0-alpha01 with the navigation view (to enable support for multiple backstacks, one per each root destination), the app bar shows the Up button for the root destinations. The expectation is that root destinations would not show the Up button.

This issue is a regression from converting class ActionBarOnDestinationChangedListener from Java to Kotlin in this commit:

https://android-review.googlesource.com/c/platform/frameworks/support/+/1560094

the method setDisplayHomeAsUpEnabled should be called with FALSE when the icon is null.

This can be reproduced by using the navigation advanced sample from the architecture components, available here:

https://github.com/android/architecture-components-samples/tree/master/NavigationAdvancedSample