List virtualization fixes

[grokys]

What does the pull request do?

Fixes some issues with the new virtualization system introduced in #9677:

• When items are removed and the list is scrolled, ensure that we cause the extent to change by adjusting the size returned from VirtualizingStackPanel.MeasureOverride. This was broken previously due to two problems:
  • We weren't setting _startUUnstable when items were removed before the realized range
  • We were clamping the stand and end indexes too late when calculating the viewport, meaning that firstIndexU was calculated with a non-clamped index, which caused the items to be realized in the wrong position
• There was a problem in ScrollContentPresenter whereby if the viewport or extent were changed the offset wasn't being coerced, leading to an offset beyond the extent of the scroll

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0029806-beta. (feed url: https://pkgs.dev.azure.com/AvaloniaUI/AvaloniaUI/_packaging/avalonia-all/nuget/v3/index.json) [PRBUILDID]

[adirh3]

@grokys maybe it's a bit off topic for this PR, but my app crashes on index out of range due toto this method giving a negative index using ItemsControl - https://github.com/AvaloniaUI/Avalonia/blob/7da9bb9d434ecb70e5ef2fc7af768067c43709f1/src/Avalonia.Controls/VirtualizingStackPanel.cs#LL491C28-L491C28

[grokys]

@adirh3 i don't suppose you have a minimal repro do you? Hard to know what's going wrong from just your description.

[adirh3]

@adirh3 i don't suppose you have a minimal repro do you? Hard to know what's going wrong from just your description.

As usual I can't repro anything outside of my app :/
The issue is that GenerateElements calls GetOrCreateElement with a negative index which cause an OutOfRangeException.
I will try to make a repro but TBH I doubt it will happen in empty app 😅

[maxkatz6]

Let's merge it, and see if somebody will find a repro for the master branch.

[avaloniaui-team]

You can test this PR using the following package version. 11.0.999-cibuild0030261-beta. (feed url: https://pkgs.dev.azure.com/AvaloniaUI/AvaloniaUI/_packaging/avalonia-all/nuget/v3/index.json) [PRBUILDID]

[grokys]

In the absence of a repro, a stacktrace might help.

[timunie]

@grokys is this maybe related? #10399 (comment)

If so, the issue seems not to be solved yet.

[adirh3]

Will give stack trace later today.
FYI I reproduced this issue using the nightly build of thus PR, so it definitely did not solve it.

[adirh3]

@grokys @maxkatz6 Sorry took longer than expected -

More debug info -

Not sure if it makes sense, but I will try to add || index < 0 to the if return condition and see if that helps without generating new bugs.