custom control not visible in designer - wpf

Question

I can't seem to see my custom control in the designer in blend or VS. I can't figure out what could be wrong. It works as expected when I run the app (e.g. I see a button)
Themes\Generic.xaml
<ResourceDictionary xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApp1">
<Style TargetType="{x:Type local:MyControl}">
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type local:MyControl}">
<StackPanel HorizontalAlignment="Left" VerticalAlignment="Top" >
<Button Content="Button" />
</StackPanel>
</ControlTemplate>
</Setter.Value>
</Setter>
</Style>
MyControl.cs
namespace WpfApp1
{
public class MyControl : Control
{
static MyControl()
{
DefaultStyleKeyProperty.OverrideMetadata(typeof(MyControl),
new FrameworkPropertyMetadata(typeof(MyControl)));
}
}
MainWindow.xaml
<Window
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:wpfApp1="clr-namespace:WpfApp1"
x:Class="WpfApp1.MainWindow"
mc:Ignorable="d"
Title="MainWindow">
<StackPanel HorizontalAlignment="Left" VerticalAlignment="Top" >
<wpfApp1:MyControl Height="100" Width="100"/>
</StackPanel>
And in assemblyInfo.cs
[assembly: ThemeInfo(ResourceDictionaryLocation.None, ResourceDictionaryLocation.SourceAssembly )]
[EDIT]
Can't explain it.
Copied all of the code into a new sln and it works as expected.
Recopied all of the code back into the original sln, removed .vs folder, rebuilt, restarted, restarted the machine, inspected the project, solution files etc... nothing it just refuses to show in one but not the other.