Native panel

[asklar]

In order to support layout/size changes coming from XAML (e.g. Expander gets expanded) and have it reflow through Yoga, we need a notification to be fired when the element's measure changes.
That notification doesn't exist but we can wrap the native XAML control in a custom Panel, where we can use the MeasureOverride override to mark the corresponding yoga node as dirty and trigger a Yoga re-layout.

However, the problem is that this panel would have its width/height set by the Yoga layout pass. As a result, XAML will not call MeasureOverride (if W/H are set, you don't need to be notified, you'll be that size!).

To work around that, we need a second outer panel, which gets the W/H set, and hosts the panel with the notification code:

<NativeHostPanel Width="200" Height="48">
  <NativeMeasuringPanel>
    <Expander />
  </NativeMeasuringPanel>
</NativeHostPanel>

the Width/Height in the outermost panel will be set by Yoga based on its desired size.

Microsoft Reviewers: Open in CodeFlow