Created Unassigned: DropDownButton without border or background does not function as expected [22497]

If you have a DropDownButton without a border or background and approach with the mouse from the right it does not focus or respond to clicks until the mouse is over the text of the button. This does not reproduce when there's a background and/or a border.

Repro code:
```
<Window x:Class="DropDownButtonDemo.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:xctk="clr-namespace:Xceed.Wpf.Toolkit;assembly=Xceed.Wpf.Toolkit"
xmlns:local="clr-namespace:DropDownButtonDemo"
mc:Ignorable="d"
Title="MainWindow"
Height="350"
Width="525">
<StackPanel Orientation="Horizontal">
<xctk:DropDownButton Width="Auto"
Height="Auto"
Content="Normal DDB (border)"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Margin="16">
<xctk:DropDownButton.DropDownContent>
<StackPanel Orientation="Vertical">
<Button Content="Button1" />
</StackPanel>
</xctk:DropDownButton.DropDownContent>
</xctk:DropDownButton>
<xctk:DropDownButton Width="Auto"
Height="Auto"
Margin="16"
Content="DDB (w/o border or background)"
HorizontalAlignment="Center"
VerticalAlignment="Center"
Background="{x:Null}"
BorderThickness="0">
<xctk:DropDownButton.DropDownContent>
<StackPanel Orientation="Vertical">
<Button Content="Button1" />
</StackPanel>
</xctk:DropDownButton.DropDownContent>
</xctk:DropDownButton>
</StackPanel>
</Window>
```