Ensure VirtualizingStackPanel correct element size estimate and fix #17460

[BobLd]

What does the pull request do?

Ensure VirtualizingStackPanel's _lastEstimatedElementSizeU is set before calling CalculateDesiredSize().

As a result, the vertical scroll bar will be correctly sized.

What is the current behavior?

The Vertical scroll bar size is not correct until the view is scroll down to its maximum.

What is the updated/expected behavior with this PR?

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?
• Consider submitting a PR to https://github.com/AvaloniaUI/avalonia-docs with user documentation

Breaking changes

n/a

Obsoletions / Deprecations

n/a

Fixed issues

Fixes #17460

[BobLd]

This is one possible solution that fixes the problem. I'll let the Avalonia experts correct me if there's a better way

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0053175-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[BobLd]

@grokys is there anything I can do here to help the review?

[avaloniaui-bot]

You can test this PR using the following package version. 11.3.999-cibuild0053787-alpha. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]