Reopened Unassigned: DropDownButton in ToolBar [22468]

I have a DropDownButton inside a ToolBar control which DropDownContent points to a CheckListBox control. First of it seems to work, but if I check/uncheck a single entry of the CheckListBox control it closes immediately. Is this a bug? How can the DropDownContent stay open?

```
<DockPanel>

<ToolBar DockPanel.Dock="Top">

<toolkit:DropDownButton Content="Click me">
<toolkit:DropDownButton.DropDownContent>
<toolkit:CheckListBox MaxHeight="400"
DisplayMemberPath="Text"
SelectedMemberPath="Enabled"
ItemsSource="{Binding Items}"/>
</toolkit:DropDownButton.DropDownContent>
</toolkit:DropDownButton>

</ToolBar>

<Grid>

</Grid>
</DockPanel>
```

Thanx in advance!