Question

So I was have a combobox like below, coded like this;

AllowsTransparency="True" Background="Transparent">
        <Border CornerRadius="10" Background="Beige" BorderBrush="Aqua" BorderThickness="2">
            <Grid>
                <StackPanel VerticalAlignment="Center" HorizontalAlignment="Center">
                    <Label Name="lblText"></Label>
                    <ComboBox Name="cbxNumbers"></ComboBox>
                    <TextBox Name="txtNumbers" Visibility="Collapsed"></TextBox>
                    <Button HorizontalAlignment="Center" Name="btnDone"
Click="btnDone_Click">That's Right!</Button>
                </StackPanel>
                <Button VerticalAlignment="Bottom" HorizontalAlignment="Right" Margin="1"
FontSize="8" Name="btnChangeInput" Click="btnChangeInput_Click">Let me chose the number
</Button>
            </Grid>
        </Border>

And then populated in code like this;

lblText.Content = Text;
cbxNumbers.Items.Add(5);
cbxNumbers.Items.Add(10);
cbxNumbers.Items.Add(50);
cbxNumbers.Items.Add(100);
cbxNumbers.Items.Add(1000);
cbxNumbers.Items.Add(10000);
cbxNumbers.Items.Add(100000);
cbxNumbers.Items.Add(1000000);
cbxNumbers.SelectedIndex = 0;

My question is obviously, why is my Dropdown showing beneath my window?

UPDATE

Quote Microsoft

Thank you for reporting this issue. Though this issue is under
investigation, we will likely not have a fix available in .NET 4.0. We
will update this bug again when we are able to fix the issue in a
future release. Thanks!

Great… any ideas for a work around?

Answer 1

This is a well documented bug with AllowTransparency=TRUE. It only happens on some computers running Win XP.

http://connect.microsoft.com/VisualStudio/feedback/details/465964/wpf-combobox-dropdown-list-appears-behind-the-form-when-allowstransparency-true

A workaround is found here:

As noted before, this is an issue with layered windows on XP. There is
not much that can be done at the WPF level to solve this, and given
that XP is no longer being serviced this isn’t very likely to be
fixed. There are potentially workarounds you can employ, though.

The source of the problem is that WPF uses something called “Layered
Windows” when WindowStyle=None and AllowsTransparency=True. Often the
reason for doing this is to implement custom window chrome. Recently
the WPF team published a library that allows you to get custom chrome
without resorting to layered windows. The library is available at

http://code.msdn.microsoft.com/WPFShell

This doesn’t support per-pixel opacity, but it does allow for
completely custom rendering of the window, including areas normally
managed by the system. It lets you set the radii of the windows
corners, but not have a completely arbitrary shape. If you can use
this library instead of AllowsTransparency=True then it should solve
this issue.

Microsoft has issued a hotfix, but it seems to not have fixed all problems related to the issue.