Grid Layout Control and ButtonDown Event

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WPF

I have the following XAML sample that should raise a PreviewMouseDown (same for MouseDown) event when clicking on the Grid background:

<Window x:Class="WpfApplication1.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="350" Width="525">

    <Grid PreviewMouseDown="Grid_PreviewMouseDown"
          HorizontalAlignment="Stretch"
          VerticalAlignment="Stretch"
          >
        <Button Margin="12,12,313,185">
            Test 23
        </Button>
    </Grid>
</Window>

When clicking onto the button, the event gets fired, but not when I click onto the grid background.

Funny enough, when I add a background color to the grid, the event will fire on a click in the background (even when I set the color to transparent):

<Grid PreviewMouseDown="Grid_PreviewMouseDown" 
    HorizontalAlignment="Stretch" 
    VerticalAlignment="Stretch"
    Background="Transparent"
    >

Is there a good reason for this behavior or is it the usual madness?

Many thanks for your help!