Fix horizontal ScrollToView in VirtualizingStackPanel in some cases

[BAndysc]

What does the pull request do?

This fixes a case when elements in VirtualizingStackPanel have different widths, ScrollToView is invoked with an element whose TargetRect in BringIntoView is out of bounds.

I am not sure if this is a big hack fix or an alright solution.

What is the current behavior?

In the BringToView ScrollViewer extents are still not updated and even thought the Offset is being set to the correct position, it is clamped to the old extents thus ScrollToView doesn't scroll to the element, see the video below:

Screen.Recording.2024-01-31.at.01.28.02.mov

What is the updated/expected behavior with this PR?

When calling BringIntoView again, the Y offset should be correct and ScrollViewer's extents should be correct, thus this call should scroll to the right horizontal position.

Checklist

• Added unit tests (if possible)?
• Added XML documentation to any related classes?

Breaking changes

n/a

Obsoletions / Deprecations

n/a

Fixed issues

Fixes #14418

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0044191-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[timunie]

I wonder if that also solves other virtualization scrolling issues, namely if items have different sizes.

[BAndysc]

I wonder if that also solves other virtualization scrolling issues, namely if items have different sizes.

Probably not, as far as I understand other virtualization scrolling issues mostly come from incorrectly estimating items height, in result scroll offset is not correctly set. In my case, Y offset was correctly set (items have same heights), but X offset wasn't because Extent was not updated yet during first BringIntoView. This is kinda a chicken-and-egg problem - in order to update width extent, the scroll offset Y needs to be set.

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0044295-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]

[avaloniaui-bot]

You can test this PR using the following package version. 11.1.999-cibuild0044473-beta. (feed url: https://nuget-feed-all.avaloniaui.net/v3/index.json) [PRBUILDID]