Avalonia/samples/ControlCatalog/Pages/ToolTipPage.xaml

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

    <StackPanel Orientation="Horizontal"
              Margin="0,16,0,0"
              HorizontalAlignment="Center"
              Gap="16">
      <Border Background="{StyleResource ThemeAccentBrush}"
              Padding="48,48,48,48">
        <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>Hover Here</TextBlock>
      </Border>
    </StackPanel>
  </StackPanel>
</UserControl>