They implicitly create a new `MediaContext` via the following stacktrack, which can break subsequent tests:
```
Avalonia.Media.MediaContext..ctor(Dispatcher dispatcher) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\Media\MediaContext.cs:line 44
at Avalonia.Media.MediaContext.get_Instance() in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\Media\MediaContext.cs:line 68
at Avalonia.Layout.LayoutManager.QueueLayoutPass() in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\Layout\LayoutManager.cs:line 356
at Avalonia.Layout.LayoutManager.InvalidateMeasure(Layoutable control) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\Layout\LayoutManager.cs:line 73
at Avalonia.Layout.Layoutable.AncestorBecameVisible(ILayoutManager layoutManager) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\Layout\Layoutable.cs:line 837
at Avalonia.Layout.Layoutable.OnPropertyChanged(AvaloniaPropertyChangedEventArgs change) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\Layout\Layoutable.cs:line 803
at Avalonia.Input.InputElement.OnPropertyChanged(AvaloniaPropertyChangedEventArgs change) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\Input\InputElement.cs:line 661
at Avalonia.Controls.Control.OnPropertyChanged(AvaloniaPropertyChangedEventArgs change) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Controls\Control.cs:line 533
at Avalonia.AvaloniaObject.OnPropertyChangedCore(AvaloniaPropertyChangedEventArgs change) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\AvaloniaObject.cs:line 649
at Avalonia.Animation.Animatable.OnPropertyChangedCore(AvaloniaPropertyChangedEventArgs change) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\Animation\Animatable.cs:line 191
at Avalonia.AvaloniaObject.RaisePropertyChanged[T](AvaloniaProperty`1 property, Optional`1 oldValue, BindingValue`1 newValue, BindingPriority priority, Boolean isEffectiveValue) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\AvaloniaObject.cs:line 700
at Avalonia.PropertyStore.EffectiveValue`1.SetAndRaiseCore(ValueStore owner, StyledProperty`1 property, T value, BindingPriority priority, Boolean isOverriddenCurrentValue, Boolean isCoercedDefaultValue) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\PropertyStore\EffectiveValue`1.cs:line 235
at Avalonia.PropertyStore.EffectiveValue`1.SetLocalValueAndRaise(ValueStore owner, StyledProperty`1 property, T value) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\PropertyStore\EffectiveValue`1.cs:line 74
at Avalonia.PropertyStore.ValueStore.SetLocalValue[T](StyledProperty`1 property, T value) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\PropertyStore\ValueStore.cs:line 220
at Avalonia.PropertyStore.ValueStore.SetValue[T](StyledProperty`1 property, T value, BindingPriority priority) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\PropertyStore\ValueStore.cs:line 196
at Avalonia.AvaloniaObject.SetValue[T](StyledProperty`1 property, T value, BindingPriority priority) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\AvaloniaObject.cs:line 336
at Avalonia.Visual.set_IsVisible(Boolean value) in D:\projects\AvaloniaUI\Avalonia\src\Avalonia.Base\Visual.cs:line 233
```
Steven Kirk