bug(overlay): BlockScrollStrategy isn't working for Overlay with flexibleConnectedTo in mat-dialog-content

[DzmVasileusky]

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

Not sure, possibly 19

Description

Recently block scroll strategy stopped working for out custom overlays used in material dialogs.
close and reposition strategies are working.
Seems to be related to OverlayPositionBuilder or flexibleConnectedTo.

Code:

function getAvailablePosition(): ConnectedPosition[] {
  return [
    {
      offsetY: 4,
      originX: 'start',
      originY: 'bottom',
      overlayX: 'start',
      overlayY: 'top',
    },
    {
      offsetY: 4,
      originX: 'end',
      originY: 'bottom',
      overlayX: 'end',
      overlayY: 'top',
    },
    {
      offsetY: -4,
      originX: 'start',
      originY: 'top',
      overlayX: 'start',
      overlayY: 'bottom',
    },
    {
      offsetY: -4,
      originX: 'end',
      originY: 'top',
      overlayX: 'end',
      overlayY: 'bottom',
    },
  ];
}

@Directive({
  host: {
    '(click)': 'toggle()',
  },
  selector: '[customDropdown]',
})
export class CustomDropdown {
  dropdownTemplateRef = input<TemplateRef<any> | undefined>();

  elementRef = inject(ElementRef);
  overlay = inject(Overlay);
  overlayPositionBuilder = inject(OverlayPositionBuilder);
  viewContainerRef = inject(ViewContainerRef);
  viewportRuler = inject(ViewportRuler);

  isOpen = false;
  overlayRef?: OverlayRef;

  getOverlayConfig(): OverlayConfig {
    const positionStrategy = this.overlayPositionBuilder
      .flexibleConnectedTo(this.elementRef)
      .withPositions(getAvailablePosition());

    const scrollStrategy = this.overlay.scrollStrategies.block();

    return {
      hasBackdrop: false,
      positionStrategy,
      scrollStrategy,
    };
  }

  hide(): void {
    if (this.overlayRef?.hasAttached()) {
      this.isOpen = false;
      this.overlayRef?.detach();
    }
  }

  show(): void {
    const templateRef = this.dropdownTemplateRef();
    if (!templateRef) {
      return;
    }
    this.overlayRef = this.overlay.create(this.getOverlayConfig());
    this.overlayRef.attach(
      new TemplatePortal(templateRef, this.viewContainerRef)
    );
    this.isOpen = true;
  }

  toggle() {
    if (!this.isOpen) this.show();
    else this.hide();
  }
}

Reproduction

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

1. Open dialog
2. Toggle dropdown
3. Scroll

Expected Behavior

Dialog isn't scrollable when overlay is opened

Actual Behavior

Dialog is scrollable when overlay is opened

Environment

• Angular: 20.0.4
• CDK/Material: 20.0.4
• Browser(s): Chrome 138.0.7204.101
• Operating System (e.g. Windows, macOS, Ubuntu): MacOS