bug(cdk-tree): aria-expanded attribute should not appear in the leaf node

[Yueyue07]

Reproduction

Example to reproduce issue
https://material.angular.io/cdk/tree/examples

Steps to reproduce:
Inspect the leaf node of cdk-tree from dev tools

Based on W3.org aria practices for tree view, it mentions that: "Each element with role treeitem that serves as a parent node has aria-expanded set to false when the node is in a closed state and set to true when the node is in an open state. End nodes do not have the aria-expanded attribute because, if they were to have it, they would be incorrectly described to assistive technologies as parent nodes."

Expected Behavior

There should be no aria-expanded attribute appearing on the leaf node

Actual Behavior

aria-expanded is setting as false on the leaf node

Environment

• CDK/Material: newest version

[chaitanya-hdoc]

@jelbourn I would like to submit a PR for this.

[nico151999]

I was reported by a screenreader user that it is quite confusing to face the expanded attribute being set on leaf elements. For accessibility reasons I would also like to see the behavior described by W3.org as mentioned by @Yueyue07

[behofer]

As a screenreader user, I can exactly see the behaviour described by @Yueyue07 consistently with different screenreaders. So, screenreaders are relying on aria-expanded being correctly set to describe tree nodes and their current states.
From an accessibility perspective, A fix would be highly appreciated!
Thanks.

[stephanie-cherba]

This is still an issue. Aria-expanded should only exist on node with children. Hopefully this can get fixed soon. Thanks!

[zarend]

Hello folks, Fix in progress, see cdk-tree-revamp branch.
-Zach