bug(MatExpansionPanel): Panel Expanded in MatStepper regardless of [expanded]

[DJayFreshBlock]

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

6

Description

Expansion panels currently render with a flash of content when nested in MatStepper -- they always temporarily appear as expanded, regardless of what the "expanded" attribute is set to. After a moment, all expansion panels that have been explicitly set to closed collapse.

Reported fixed by #13870
However this is just when opened in a dialog without any other nesting

Reproduction

StackBlitz link: https://angular-issue13870-stepper-dialog.stackblitz.io
Steps to reproduce:

1. Create a MatStepper component to contain an expansion panel
2. Add a MatExpansionPanel
3. Set [expanded]="false"

Expected Behavior

MatExpansionPanels currently render with a flash of content when nested -- they always temporarily appear as expanded, regardless of what the "expanded" attribute is set to. After a moment, all expansion panels that have been explicitly set to closed collapse.

Actual Behavior

MatExpansionPanels should appear as collapsed when the dialog opens when expanded=false instead of briefly appearing as expanded, then collapsing after a delay.

Environment

• Angular: 14.2
• CDK/Material: 14.2
• Browser(s): Chrome
• Operating System : Windows

[DJayFreshBlock]

@crisbeto Thanks for the quick turn around on this issue. Assuming this will only be fixed in the next major release?

[crisbeto]

The fix should be in 16.1.2 later this week.

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