bug(mat-button): mat-button should be changed to disabled appearance when the parent `<fieldset>` is disabled

[mrpachara]

Is this a regression?

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

The previous version in which this bug was not present was

No response

Description

The same issue #3033 was closed by angular/angular#35309. But it only fixes form controls not buttons.

The CSS selector for disabled appearance only selects [disabled] but it should also select :disabled to make the parent <fieldset disabled> work.

Reproduction

StackBlitz link: https://stackblitz.com/edit/components-issue-starter-ck6fmjkd
Steps to reproduce:

1. Put both mat-button and disabled-mat-button in <fieldset disabled>.

Expected Behavior

Both mat-button should show the same appearance.

Actual Behavior

They are different.

Environment

• Angular: 20.0.6
• CDK/Material: 20.0.5
• Browser(s): Google Chrome
• Operating System (e.g. Windows, macOS, Ubuntu): Windows