White square on area with scrollbar - wpf

Question

I have a custom Scroll bar style, but it has this annoying little white square. I want to remove it, any ideas how?
Annoying Little White Square:
Edit: it might be a repeat button, I think

Answer 1

The problem is clearly related to your style.
And how to answer your question without seeing this style?
Based on my attempt to guess what you might have in the style, but I assume that you do not change the default ScrollViewer template, you only set values for its properties.
If you look at the default template, there is a rectangle x:Name="Corner" in the corner between the ScrollBar's.
<ControlTemplate x:Key="ScrollViewerControlTemplate.Default"
TargetType="{x:Type ScrollViewer}">
<Grid x:Name="Grid" Background="{TemplateBinding Background}">
<Grid.ColumnDefinitions>
<ColumnDefinition Width="*"/>
<ColumnDefinition Width="Auto"/>
</Grid.ColumnDefinitions>
<Grid.RowDefinitions>
<RowDefinition Height="*"/>
<RowDefinition Height="Auto"/>
</Grid.RowDefinitions>
<Rectangle x:Name="Corner"
Grid.Column="1" Grid.Row="1"
Fill="{DynamicResource {x:Static SystemColors.ControlBrushKey}}"/>
<ScrollContentPresenter x:Name="PART_ScrollContentPresenter"
Grid.Column="0" Grid.Row="0"
CanContentScroll="{TemplateBinding CanContentScroll}"
CanHorizontallyScroll="False" CanVerticallyScroll="False"
ContentTemplate="{TemplateBinding ContentTemplate}"
Content="{TemplateBinding Content}"
Margin="{TemplateBinding Padding}"/>
<ScrollBar x:Name="PART_VerticalScrollBar"
Grid.Column="1" Grid.Row="0"
AutomationProperties.AutomationId="VerticalScrollBar"
Cursor="Arrow"
Maximum="{TemplateBinding ScrollableHeight}" Minimum="0"
Visibility="{TemplateBinding ComputedVerticalScrollBarVisibility}"
Value="{Binding VerticalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}"
ViewportSize="{TemplateBinding ViewportHeight}"/>
<ScrollBar x:Name="PART_HorizontalScrollBar"
AutomationProperties.AutomationId="HorizontalScrollBar"
Cursor="Arrow"
Grid.Column="0" Grid.Row="1"
Maximum="{TemplateBinding ScrollableWidth}"
Minimum="0"
Orientation="Horizontal"
Visibility="{TemplateBinding ComputedHorizontalScrollBarVisibility}"
Value="{Binding HorizontalOffset, Mode=OneWay, RelativeSource={RelativeSource TemplatedParent}}"
ViewportSize="{TemplateBinding ViewportWidth}"/>
</Grid>
</ControlTemplate>
The background color of this rectangle is set by DynamicResource with key {x:Static SystemColors.ControlBrushKey}.
Therefore, all you can do without changing the template is to create a new resource with the same key, but with a different color.
The resource can be set in the same place where you set your style.
An example of defining this resource:
<SolidColorBrush x:Key="{x:Static SystemColors.ControlBrushKey}"
Color="Red"/>
If you do change the template, then you can simply delete this rectangle, then this corner will be the background color of the ScrollViewer.
You can also, after removing it, stretch one of the ScrollBars to this corner.