AutoCompleteBox: Setting MinimumPrefixLength to 0 causes dropdown to open when control is shown for the first time

[MarchingCube]

AutoCompleteBox with MinimumPrefixLength opens on it's own when show for the first time. Simplest repro is setting MinimumPrefixLength to 0 for any of the controls in AutoCompleteBoxPage.xaml.

In example below I have set it to all controls, which opens dropdowns for them all. And it requires a click per each to close them.

Sample below simply wraps AutoCompleteBox in a Expander. For the first time Expander is opened the dropdown will show, subsequent opens do not trigger a dropdown open.

<ListBox Items="{Binding TestItems}">
    <ListBox.ItemTemplate>
      <DataTemplate>
        <Expander Header="Test Element">
            <AutoCompleteBox Items="{Binding CompleteItems}" MinimumPrefixLength="0" FilterMode="ContainsOrdinal"/>
        </Expander>
      </DataTemplate>
    </ListBox.ItemTemplate>
  </ListBox>

cc: @sdoroff since he ported this control from Silverlight and maybe can shed some light on this behavior.

[sdoroff]

@MarchingCube - this is clearly bug. I'll hopefully have a fix for it this week. Thank you for pointing it out.