LiveAnnouncer does not work when made inside a mat-dialog

[dlpx-gli]

Reproduction

Use StackBlitz to reproduce your issue:
https://stackblitz.com/edit/components-issue-fksfhp?file=src/app/dialog-content-example.ts

Steps to reproduce:

1. Open screen reader, I used mac voiceover. Everything below are done via screen reader.
2. Click on the "Open dialog" button to open dialog
3. Inside the dialog, click the "test" button which would make a live announcement.
4. It says "press test, button". No announcement made. If I open the inspector, the cdk-live-announcer-element region is correctly populated with the announcement text despite no actual announcement. In addition, if I move this div inside <mat-dialog-container> it would work but then this element becomes reacheable via screen reader.

Expected Behavior

What behavior were you expecting to see?

Make the announcement.

Actual Behavior

What behavior did you actually see?

No announcement even though cdk-live-announcer-element did get populated.

Environment

• Angular: "10.2.4"
• CDK/Material: "10.2.7"
• Browser(s): Chrome Version 90.0.4430.93
• Operating System (e.g. Windows, macOS, Ubuntu): macOS mojave 10.14.6

[dlpx-gli]

The problem seems to be specific to chrome. It works fine for firefox.

[crisbeto]

What screen reader is this on? It worked correctly for me with NVDA on Chrome.

[dlpx-gli]

What screen reader is this on? It worked correctly for me with NVDA on Chrome.

This was the voiceover on macOS mojave 10.14.6

[crisbeto]

If that's the case, I suspect that there may be issues with the LiveAnnouncer outside of a dialog as well, because the two are basically the same. I'll mark this as "needs investigation" so that we can verify it against VoiceOver.

[zelliott]

Without having done any investigation into this, it could be due to the dialog's focus trap implementation. I wonder if the LiveAnnouncer element is having aria-hidden="true" applied, or if the existence of aria-modal="true" on the dialog is preventing SRs from announcing the LiveAnnouncer element.