[Prototype][DNM]Adjust spinner vertical position

[duncanuszkay-d2l]

https://desire2learn.atlassian.net/browse/NTNGL-5380

If you trigger the table loading state after scrolling, you won't see the loading spinner at all since it's statically positioned below the top of the element.

In this PR, I'm calculating the vertical position for the spinner each time the loading backdrop is enabled. The position is equal to the center of the visible portion of the element.

To avoid excessive recalculation and visual noise, we do not recalculate this position again until the next time the backdrop is triggered, even if the user then scrolls or resizes the page.

Functional Testing

Testing the behaviour in three scenarios:

1. The viewport contains the element completely
2. The element ends below the viewport
3. The element starts above the viewport

Recording.2026-02-19.165349.mp4

4. The element starts above and ends below the viewport

Recording.2026-02-19.165631.mp4

[github-actions]

Thanks for the PR! 🎉

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[duncanuszkay-d2l]

This makes the demo long enough to easily test this property

[duncanuszkay-d2l]

I'm using a private property here to indicate that the spinner should be repositioned after the next render- the flow looks like this:

1. The user marks the element as 'shown'
2. We call show(), which sets the state properties to start the fade-in, and records that we'll need to re-position after this render
3. We render the page as a result of the properties changing (must occur first to get page dimensions before we can center)
4. When we move onto updated we see that the spinner needs to be re-positioned, so we do so and remove the mark
5. Future renderings, such as those that result from transitions starting/completing, don't trigger further repositioning of the spinner, even if the viewport has since moved. This is by design.

[dlockhart]

I think you could do without it and check for changedProperties having _state being set to either shown (if reduced motion) or showing (if not reduced motion)?