How can I make sure that a GridSplitter Is visible ?

To prevent hidden GridSplitter controls, do one of the following :

[XAML]

<Grid>
  <Grid.ColumnDefinitions>
    <ColumnDefinition/>
    <ColumnDefinition/>
  </Grid.ColumnDefinitions>
  <Button Grid.Column='0'/>
  <GridSplitter Grid.Column ='0' Background='Blue'/>
</Grid>

[XAML]

<Grid>
  <Grid.ColumnDefinitions>
    <ColumnDefinition/>
    <ColumnDefinition/>
  </Grid.ColumnDefinitions>
  <GridSplitter Grid.Column='0' Background='Blue'
                Panel.ZIndex='1'/>
  <Button Grid.Column='0'/>
</Grid>

[XAML]

<Grid>
  <Grid.ColumnDefinitions>
    <ColumnDefinition/>
    <ColumnDefinition/>
  </Grid.ColumnDefinitions>
  <GridSplitter Grid.Column ='0' Background='Blue'/>
  <Button Grid.Column='0' Margin='0,0,5,0'/>
</Grid>