bug(virtual-scroll): scrollToIndex on viewport while inside cdkVirtualScrollingElement is wrong

[Harpush]

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

When scrolling by index using scrollToIndex on the viewport it doesn't take into account any content around the viewport when using cdkVirtualScrollingElement.
The logic calculates the offset only based on the itemSize without taking into account the viewport offset.

Reproduction

StackBlitz link: https://stackblitz.com/edit/qmx1v9bc
Steps to reproduce:

1. In the Regular section clicking on the button Scroll to item 50 correctly scrolls to item 50. This is a regular viewport without cdkVirtualScrollingElement.
2. In the With cdkVirtualScrollingElement section clicking on the button Scroll to item 50 (not working) it scrolls to item 48 instead of 50 as there is a 100px header above the viewport while it should scroll to item 50.
3. If the button Scroll to item 50 (fixed) is clicked the scrolling works as expected as I calculate the offset myself using viewport.measureViewportOffset('top') which is missing in scrollToIndex.

Expected Behavior

It should scroll to the correct item when using cdkVirtualScrollingElement

Actual Behavior

It scrolls without taking into account the viewport offset.

Environment

• Angular: 21.2.6
• CDK/Material: 21.1.2
• Browser(s): Chrome 146
• Operating System (e.g. Windows, macOS, Ubuntu): Windows 11