bug(CdkVirtualScrollViewport): no unsubscribe from scrolling after destroy

[RealOFF]

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

CdkVirtualScrollViewport subscribes to this.scrollable.elementScrolled() but does not unsubscribe from it. In most cases, when you are not using cdkVirtualScrollingElement, this will not be any problem because scrollable unsubscribes when destroyed, but when we use cdkVirtualScrollingElement and the cdk-virtual-scroll-viewport element is mounted/unmounted from the DOM, there will be unnecessary scroll subscriptions. This subscriptions can lead to some unexpected behaviours.

Reproduction

StackBlitz link: https://stackblitz.com/edit/l1quzt-cwdqhc?file=src%2Fexample%2Fcdk-virtual-scroll-parent-scrolling-example.html
Steps to reproduce:

1. Click a lot of time on "Toggle show" button.
2. Setup breakpoint in FixedSizeVirtualScrollStrategy.prototype.onContentScrolled(vendor.js 22253 line).
3. Scroll content to the bottom.

Expected Behavior

After small scroll the onContentScrolled should be called only one time with this._viewport in FixedSizeVirtualScrollStrategy that equal CdkVirtualScrollViewport instance.

Actual Behavior

After small scroll the FixedSizeVirtualScrollStrategy.prototype.onContentScrolled will be called a lot of time with this._viewport === null in FixedSizeVirtualScrollStrategy.

Environment

• Angular: 16.2.3
• CDK/Material: CDK
• Browser(s): Chrome
• Operating System (e.g. Windows, macOS, Ubuntu): macOS

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