Avalonia/samples/ControlCatalog/Pages/ToolTipPage.xaml

<UserControl xmlns="https://github.com/avaloniaui">
    <StackPanel Orientation="Vertical"
                Spacing="4">
        <TextBlock Classes="h1">ToolTip</TextBlock>
        <TextBlock Classes="h2">A control which pops up a hint when a control is hovered</TextBlock>

        <Grid RowDefinitions="Auto,Auto"
              ColumnDefinitions="Auto,Auto"
              Margin="0,16,0,0"
              HorizontalAlignment="Center">
            <Border Grid.Column="0"
                    Grid.Row="1"
                    Background="{DynamicResource ThemeAccentBrush}"
                    Margin="5"
                    Padding="50"
                    ToolTip.Tip="This is a ToolTip">
                <TextBlock>Hover Here</TextBlock>
            </Border>
            <CheckBox Grid.Column="1"
                      Margin="5"
                      Grid.Row="0"
                      IsChecked="{Binding ElementName=Border, Path=(ToolTip.IsOpen)}"
                      Content="ToolTip Open" />
            <Border Name="Border"
                    Grid.Column="1"
                    Grid.Row="1"
                    Background="{DynamicResource ThemeAccentBrush}"
                    Margin="5"
                    Padding="50"
                    ToolTip.Placement="Bottom">
                <ToolTip.Tip>
                    <StackPanel>
                        <TextBlock Classes="h1">ToolTip</TextBlock>
                        <TextBlock Classes="h2">A control which pops up a hint when a control is hovered</TextBlock>
                    </StackPanel>
                </ToolTip.Tip>
                <TextBlock>ToolTip bottom placement</TextBlock>
            </Border>
        </Grid>
    </StackPanel>
</UserControl>