bug(mat-tree): renderNodeChanges sets all tabindices to "-1"

[aHereitani]

Is this a regression?

• Yes, this behavior used to work in the previous version

Description

Using CdkTree.renderNodeChanges (or reassigning CdkTree.dataSource) leads to all tabindices of mat-tree-node being set to "-1", making them unavailable for keyboard navigation.

Reproduction

StackBlitz link:
Steps to reproduce:

1. visit https://stackblitz.com/edit/sqa8lvn4
2. hit "rerender" button

Expected Behavior

First node has tabindex="0" in order to be reachable by tabbing.

Actual Behavior

All nodes are removed from taborder.

Environment

• Angular: 21
• CDK/Material: 21
• Browser(s): tested in Chrome and Firefox
• Operating System (e.g. Windows, macOS, Ubuntu): tested on Windows