ScrollViewer.ScrollToEnd() seems not working

Describe the bug

ScrollViewer.ScrollToEnd() method is not working, content still remains in the same position.

To Reproduce Steps to reproduce the behavior:

1. Create a ScrollViewer
2. Add some vertical contents to activate the vertical scroll bar
3. Call ScrollViewer.ScrollToEnd()
4. See the error

Expected behavior

ScrollViewer should go to the end of the content.

Screenshots

ScrollViewer’s style:

<Style Selector="ScrollViewer">
        <Setter Property="Background"
                Value="Transparent" />
        <Setter Property="Template">
            <ControlTemplate>
                <Grid ColumnDefinitions="*,Auto" RowDefinitions="*,Auto">
                    <ScrollContentPresenter Name="PART_ContentPresenter"
                                            Grid.Row="0"
                                            Grid.Column="0"
                                            Grid.RowSpan="2"
                                            Grid.ColumnSpan="2"
                                            Background="{TemplateBinding Background}"
                                            CanHorizontallyScroll="{TemplateBinding CanHorizontallyScroll}"
                                            CanVerticallyScroll="{TemplateBinding CanVerticallyScroll}"
                                            Content="{TemplateBinding Content}"
                                            Extent="{TemplateBinding Extent, Mode=TwoWay}"
                                            Margin="{TemplateBinding Padding}"
                                            Offset="{TemplateBinding Offset, Mode=TwoWay}"
                                            Viewport="{TemplateBinding Viewport, Mode=TwoWay}">
                        <ScrollContentPresenter.GestureRecognizers>
                            <ScrollGestureRecognizer
                                CanHorizontallyScroll="{TemplateBinding CanHorizontallyScroll}"
                                CanVerticallyScroll="{TemplateBinding CanVerticallyScroll}" />
                        </ScrollContentPresenter.GestureRecognizers>
                    </ScrollContentPresenter>
                    <ScrollBar Name="PART_HorizontalScrollBar"
                               AllowAutoHide="{TemplateBinding AllowAutoHide}"
                               Orientation="Horizontal"
                               LargeChange="{Binding LargeChange.Width, RelativeSource={RelativeSource TemplatedParent}}"
                               SmallChange="{Binding SmallChange.Width, RelativeSource={RelativeSource TemplatedParent}}"
                               Maximum="{TemplateBinding HorizontalScrollBarMaximum}"
                               Value="{TemplateBinding HorizontalScrollBarValue, Mode=TwoWay}"
                               ViewportSize="{TemplateBinding HorizontalScrollBarViewportSize}"
                               Visibility="{TemplateBinding HorizontalScrollBarVisibility}"
                               Grid.Row="1"
                               Focusable="False" />
                    <ScrollBar Name="PART_VerticalScrollBar"
                               AllowAutoHide="{TemplateBinding AllowAutoHide}"
                               Orientation="Vertical"
                               LargeChange="{Binding LargeChange.Height, RelativeSource={RelativeSource TemplatedParent}}"
                               SmallChange="{Binding SmallChange.Height, RelativeSource={RelativeSource TemplatedParent}}"
                               Maximum="{TemplateBinding VerticalScrollBarMaximum}"
                               Value="{TemplateBinding VerticalScrollBarValue, Mode=TwoWay}"
                               ViewportSize="{TemplateBinding VerticalScrollBarViewportSize}"
                               Visibility="{TemplateBinding VerticalScrollBarVisibility}"
                               Grid.Column="1"
                               Focusable="False" />
                    <Panel x:Name="PART_ScrollBarsSeparator" Grid.Row="1" Grid.Column="1"
                           Background="{DynamicResource ScrollViewerScrollBarsSeparatorBackground}" />
                </Grid>
            </ControlTemplate>
        </Setter>
    </Style>

Desktop (please complete the following information):

• OS: Windows 11 22H2
• Version: Latest Avalonia nightly build

[grokys]

Without an actual repro it’s difficult to say for sure, but indeed in your case it sounds like you will need a layout pass to complete before being able to scroll to the end. That’s not a bug, it’s simply how it works - the contents of the ItemsControl are updated on layout.

That doesn’t mean that all cases of scrolling to the end require a layout pass however, so I think the PR should be closed.

[laolarou726]

There aren’t an examples of where you’re calling ScrollToEnd here so it’s hard to say what’s going wrong, but I assume you’re calling it in the constructor or something?

In this case, @maxkatz6 is correct that a layout pass would be needed before ScrollToEnd can work.

I think, default API can be changed to async and be called on the dispatcher automatically.

However I disagree with this, there are a lot of cases where you don’t need to do this (the majority in fact).

Basically I have an ItemsControl in the ScrollViewer.

I’m calling ScrollViewer.ScrollToEnd() in the event where the items source of ItemsControl is changed. So it should be in the UI thread. If I only use ScrollToEnd() without Dispatcher priority the content will not refresh. So maybe is a bug or something related to virtualization?