I encountered an issue with the Avalonia Previewer where Unicode characters (specifically U+1F5D5) were not rendered properly. The Previewer would not display correctly when trying to use this character. After some investigation, I found that the issue was related to the default font used by the Previewer, which did not support this Unicode character. To fix the issue, I applied the following style to the Window:

<Window.Styles>
    <Style Selector="Window">
        <Setter Property="FontFamily" Value="Segoe UI" />
        <Setter Property="FontSize" Value="12" />
    </Style>
</Window.Styles>

This resolved the issue, and the Unicode characters displayed correctly in the Previewer. I hope this can help others facing similar problems!