isTransparencyModeEnabled ignored for disabled PopupButton

As per title, PopupButton.isTransparencyModeEnabled only works if the control is enabled. Once disabled, the borders etc render again.

If anyone else runs into the problem, a (bad) workaround till fixed is to override the Control Template, and in the trigger for isEnabled, add something like:

<Setter Property="BackgroundDisabled" TargetName="contentArea" Value="Transparent"/>

<Setter Property="BorderBrushDisabled" TargetName="contentArea" Value="Transparent"/>

<Setter Property="BackgroundDisabled" TargetName="PART_IndicatorArea" Value="Transparent"/>

<Setter Property="BorderBrushDisabled" TargetName="PART_IndicatorArea" Value="Transparent"/>