bug(sidenav): nested side nav, inner fixedinviewport not respected in webkit

[AngelaM-C]

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

Situation:
When you have nested sidenavs and the inner sidenav mode is over and fixedInViewport is true that is not being respected properly in webkit browsers.

Workaround:
As a workaround, we found that setting the z-index of the outer container and content to auto fixes the issue.

Reproduction

StackBlitz link: link

Steps to reproduce:

1. open stackblitz in chrome to see expected behavior
2. toggle the inner drawer and see that the content is rendered over the toolbar(sibling content)
3. open stackblitz in safari or webkit
4. toggle the inner drawer and see that the content is rendered under the toolbar(sibling content)
5. uncomment the css to see the workaround

Expected Behavior

It should work in webkit as it does in chrome. Fixedinviewport and over mode should not only render on top of parent content but also any content that is sibling of the parent.

Actual Behavior

On webkit, the inner sidenav is rendered underneath any sibling content of the outer sidenav.

Environment

• Angular:19.0.3
• CDK/Material:19.0.3
• Browser(s): Safari, Webkit
• Operating System (e.g. Windows, macOS, Ubuntu): MacOS