DropDownButton closes when nested Popup closes

[bozalina]

If you place a Popup, such as one from a ComboBox, inside a DropDownButton and click outside the nested Popup, the DropDownButton's Popup closes as well.

To reproduce, place a ComboBox inside the DropDownButton sample in the explorer:

<xctk:DropDownButton x:Name="_dropDownButton"
                               Grid.Row="1"
                               Margin="10"
                               Width="150"
                               HorizontalAlignment="Left"
                               Content="Click Me"
                               MaxDropDownHeight="{Binding Value, ElementName=maxDropDownHeight}"
                               DropDownPosition="{Binding SelectedItem, ElementName=dropDownPosition}"
                               DropDownContentBackground="{Binding SelectedColor, ElementName=dropDownContentBackground, Converter={StaticResource ColorToSolidColorBrushConverter}}"
                               Click="DropDownButton_Click">
            <xctk:DropDownButton.DropDownContent>
              <StackPanel>
                <xctk:ColorCanvas Margin="10"/>
                  <ComboBox>
                    <ComboBox.Items>
                      <sys:String>Item</sys:String>
                    </ComboBox.Items>
                  </ComboBox>
                <Button Click="Button_Click"
                        Margin="10">OK</Button>
              </StackPanel>
            </xctk:DropDownButton.DropDownContent>
          </xctk:DropDownButton>

Run the explorer and open the ComboBox. Click outside the ComboBox's Popup, but within the DropDownButton's PopUp. Observe that the DropDownButton closes as well. It seems to be caused by the PreviewMouseDownOutsideCapturedElement event tunneling through the DropDownButton's Popup to the ComboBox's Popup.

[XceedBoucherS]

Thank you for reporting this.
The fix will be included in v3.8.