Avalonia/src/Avalonia.Themes.Fluent/Controls/ToolTip.xaml

<ResourceDictionary xmlns="https://github.com/avaloniaui"
                    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
                    xmlns:sys="using:System"
                    x:ClassModifier="internal">
  <Design.PreviewWith>
    <Border Padding="20">
      <StackPanel Spacing="20">
        <ToolTip Opacity="1">Text Content</ToolTip>
        <ToolTip Opacity="1">Very long text content which should exceed the maximum with of the tooltip and wrap.</ToolTip>
        <ToolTip Opacity="1">
          <StackPanel>
            <TextBlock>Multi-line</TextBlock>
            <TextBlock>Control Content</TextBlock>
          </StackPanel>
        </ToolTip>
      </StackPanel>
    </Border>
  </Design.PreviewWith>

  <sys:Double x:Key="ToolTipContentMaxWidth">320</sys:Double>

  <ControlTheme x:Key="{x:Type ToolTip}" TargetType="ToolTip">
    <Setter Property="Foreground" Value="{DynamicResource ToolTipForeground}" />
    <Setter Property="Background" Value="{DynamicResource ToolTipBackground}" />
    <Setter Property="BorderBrush" Value="{DynamicResource ToolTipBorderBrush}" />
    <Setter Property="BorderThickness" Value="{DynamicResource ToolTipBorderThemeThickness}" />
    <Setter Property="FontSize" Value="{DynamicResource ToolTipContentThemeFontSize}" />
    <Setter Property="Padding" Value="{DynamicResource ToolTipBorderThemePadding}" />
    <Setter Property="MaxWidth" Value="{DynamicResource ToolTipContentMaxWidth}" />
    <Setter Property="CornerRadius" Value="{DynamicResource OverlayCornerRadius}" />
    <Setter Property="Opacity" Value="0" />
    <Setter Property="Transitions">
      <Transitions>
        <DoubleTransition Property="Opacity" Duration="0:0:0.15" />
      </Transitions>
    </Setter>
    <Setter Property="Template">
      <ControlTemplate>
        <Border Name="PART_LayoutRoot"
                BorderThickness="{TemplateBinding BorderThickness}"
                Background="{TemplateBinding Background}"
                BorderBrush="{TemplateBinding BorderBrush}"
                Padding="{TemplateBinding Padding}"
                CornerRadius="{TemplateBinding CornerRadius}">
          <ContentPresenter Name="PART_ContentPresenter"
                            MaxWidth="{TemplateBinding MaxWidth}"
                            Content="{TemplateBinding Content}"
                            ContentTemplate="{TemplateBinding ContentTemplate}"
                            TextBlock.TextWrapping="Wrap"/>
        </Border>
      </ControlTemplate>
    </Setter>

    <Style Selector="^:open">
      <Setter Property="Opacity" Value="1" />
    </Style>
  </ControlTheme>
</ResourceDictionary>