I have a small user control with ‘Browse’ button for selecting some file and with textbox for visualization of path to selected file:
<TextBox Text="{Binding FilePath, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" />
<Button Content="Browse" cal:Message.Attach="SelectInstallationPointsFile" />
The control starting from window dialog as window with two buttons – OK and Cancel that defined on window resourse style (simplified version of the code):
<Style TargetType="{x:Type Window}">
...
<Setter Property="Template">
<Setter.Value>
<ControlTemplate TargetType="{x:Type Window}">
...
<ContentControl Content="{TemplateBinding Content}" ContentTemplate="{TemplateBinding ContentTemplate}" /> <!-- here will be the user control -->
<Button IsDefault="True" Content="OK" cal:Message.Attach="Apply" />
<Button IsCancel="True" cal:Message.Attach="Cancel" />
...
</Style>
View of dialog (green section – the window, red – the user control):

The problem is next – when you click the Enter button, then react the Browse button from user control, but not OK button from window dialog, although OK button has IsDefault="True"!
So, I just tried Focusable=”False” in the ‘Browse’ button, and it works!)