WPF, Whats with this weird whitespace in Tooltip/Border

Question

I have styled a tooltip like

<Setter Property="ToolTip">
    <Setter.Value>
        <Border Background="Red" CornerRadius="5" Padding="5" Margin="0">
            <TextBlock Text="Hello" Foreground="White" />
        </Border>
    </Setter.Value>
</Setter>

And it gets rendered like

Whats that white "rectangle" about? How can I get rid of it

Answer 1

That rectangle comes from the ControlTemplate of the ToolTip control wich will contain whatever you set in Control.ToolTip. You have to override it:

<Style TargetType="ToolTip">
    <Setter Property="Template">
        <Setter.Value>
            <ControlTemplate TargetType="ToolTip">
                <ContentPresenter />
            </ControlTemplate>
        </Setter.Value>
    </Setter>
</Style>

Answer 2

Have you tried reducing the padding of the tooltip directly:

<ToolTip Padding="0" >
...
</Tooltip>