bug(MAT TAB): Mat-tab-group dont change properly in Lazy Loading Mode & selectedIndex

[danielschmitz]

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

material 19.1.5

Description

im using a Mat-Tab-Group with <ng-template matTabContent> to create a wizard (lazy), but the MatTab.selectedIndex don't work properly.

Example:
https://stackblitz.com/~/github.com/danielschmitz/tabs-as-wizard
demo: https://danielschmitz.github.io/tabs-as-wizard/
code: https://github.com/danielschmitz/tabs-as-wizard

Click "Next" Then "Back" ... dont works... but if click at radio.. the page back to 0 :-(

See

My simple code:

a mat-tab with selectedIndex

<mat-tab-group [(selectedIndex)]="selectedIndex">
  <mat-tab label="Step 1">
    <ng-template matTabContent>
    <mat-card>
      <mat-card-title>Step 1 - Basic Info</mat-card-title>
      <mat-card-content>
            ..... form........
      </mat-card-content>
      <mat-card-actions>
        <button mat-stroked-button color="primary" (click)="prevTab()" [disabled]="isFirst">Back</button>
        <span class="spacer"></span>
        <button mat-raised-button color="primary" (click)="nextTab()" [disabled]="isLast">Next</button>
      </mat-card-actions>

The prevTab code:

nextTab(): void {
    if (!this.isLast) this.selectedIndex++;
  }

  prevTab(): void {
    if (!this.isFirst) this.selectedIndex--;
  }

Angular 19 & 20

Reproduction

StackBlitz link: https://stackblitz.com/~/github.com/danielschmitz/tabs-as-wizard
Steps to reproduce:

1. Click Next Button
2. In the Step 2, click Back (dont works)
3. Click in the Radio Phone... page go back

Expected Behavior

1. Click Next Button
2. In the Step 2, click Back
3. Page go back to Step 1

Actual Behavior

1. Click Next Button
2. In the Step 2, click Back (dont works)
3. Click in the Radio Phone... page go back

Environment

• Angular: 19.2.0
• CDK/Material: 19.2.19
• Browser(s): Chrome, Edge, Brave
• Operating System (e.g. Windows, macOS, Ubuntu): Windows

[mmalerba]

This is a bug in the demo app, you're not marking for check or using a signal for selectedIndex. Switching it to a signal makes the tabs behave as normal