Cannot set a popup's RenderTransform

[cboittin]

Describe the bug

When setting the RenderTransform of a popup, nothing happens.
The avalonia devtools debugger shows a FormatException trying to set the property. The devtools do seem to understand the transform however, because it's applied in the overlay when hovering over the popup in the visual tree.

This happens both when setting the transform with a string (e.g. "translateY(100px)") or when directly using a TranslateTransform xaml element. Also if setting it directly from code.

To Reproduce

In your mainwindow:

<Popup Width="600" Height="400" PlacementMode="Top" IsOpen="True">
    <Popup.RenderTransform>
        <TranslateTransform Y="100"/>
    </Popup.RenderTransform>
    <Border Background="Pink"/>
</Popup>

Expected behavior

No response

Avalonia version

11.3.9

OS

No response

Additional context

No response