Scrolling a ListBox with differing item height with virtualization enabled messes up the arrange of the selected item

[tvass83]

Describe the bug

This is a regression b/w 0.10.22 .. 11.2.3.

Seems that the selected item is cached (in _focusedElement) and is not recycled as regular items that leave the viewport.

Avalonia/src/Avalonia.Controls/VirtualizingStackPanel.cs

Lines 224 to 232 in 8fe22dc

	// Ensure that the focused element is in the correct position.
	if (_focusedElement is not null && _focusedIndex >= 0)
	{
	u = GetOrEstimateElementU(_focusedIndex);
	var rect = orientation == Orientation.Horizontal ?
	new Rect(u, 0, _focusedElement.DesiredSize.Width, finalSize.Height) :
	new Rect(0, u, finalSize.Width, _focusedElement.DesiredSize.Height);
	_focusedElement.Arrange(rect);
	}

Then in VirtualizingStackPanel.Arrange(), the cached item is also arranged but with incorrectly calculated finalSize.

Avalonia/src/Avalonia.Controls/VirtualizingStackPanel.cs

Lines 648 to 651 in 8fe22dc

	var estimatedSize = EstimateElementSizeU();
	// TODO: Use _startU to work this out.
	return index * estimatedSize;

The calculation currently just averages the desired height of the realized containers, that only works fine when using identical item height values.

To Reproduce

Check the following repro apps to see the different behavior in Avalonia 0.10.22 vs 11.2.3.
There's also a WPF app here that works as expected.
https://github.com/tvass83/AvaloniaIssues/tree/main/InvalidBoundsForSelectedItem

Expected behavior

The selected item shall not randomly appear on screen while scrolling through the items.

Avalonia version

11.2.3

OS

No response

Additional context

Considered the following issues & PRs before filing this issue:
#9269
#2144
#10367
#17349
#10892

[timunie]

Hey @tvass83 ,

Since you have already started to investigate the root cause, feel free to provide a PR in case you have an idea how to fix the measurement.

[tvass83]

Seems like @grokys is the expert in this area. Steven, do you think there could be a simple fix for this issue?
I understand that the term simple & virtualization are not best friends though... 😂