I have a window that looks like this:

<Window x:Class="MyWindow"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    Title="My Title" Height="300" Width="300" ResizeMode="NoResize" SizeToContent="Height" WindowStartupLocation="CenterOwner">
    <DockPanel>
        <Grid x:Name="MyGrid" DockPanel.Dock="Top" Margin="10">

        </Grid>
    </DockPanel>
</Window>

Pretty simple. The instructions I am following suggest that if I click the item, the properties window will appear (it does), and then I can click ColumnDefinitions (which is a Collection) to bring up the Collection Editor and add some columns.

However, the ColumnDefinitions property looks like this:

The ColumnDefinitions ellipsis you'd normally press is gone, or invisible, or something.

Any idea why?