bug(autocomplete): mat-mdc-autocomplete-panel position change breaks nested absolute positioning

[JuanGoria]

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

19.2.19

Description

After Angular Material v20, the mat-mdc-autocomplete-panel default positioning changed from position: static to position: relative.
PR
This change breaks the layout of nested elements that use position: absolute with positioning relative to the autocomplete panel's parent container. Previously, nested absolutely-positioned elements (like a side list) would render outside the autocomplete panel. Now they render constrained within it.

Questions:

• Was this intended?
• Should I change the implementation of nested list to adapt to it? It can be fixed via updating the global styling to position: static again

Reproduction

StackBlitz link: https://stackblitz.com/edit/stackblitz-starters-gbhzljrw?file=src%2Fmain.ts
Steps to reproduce:

1. Open the list, check the secondary list is rendered inside the autocomplete
2. Switch between position relative and static, the list is rendered outside the autocomplete (static)

Expected Behavior

Nested elements with position: absolute should render outside the autocomplete panel's bounds, positioned relative to the nearest non-static ancestor (the panel's parent container) instead of .cdk-overlay-pane, allowing side-by-side layouts as originally designed.

Actual Behavior

Nested elements with position: absolute now render inside the autocomplete panel's bounds because position: relative on the panel creates a new stacking context, constraining absolute positioning to the panel itself rather than .cdk-overlay-pane.

Environment

• Angular: 21.1.0
• CDK/Material: 21.1.5
• Browser(s): Chrome
• Operating System (e.g. Windows, macOS, Ubuntu): Windows 11