The opacity in the Grid named "Src" is causing this artifact.

As a workaround you can move that opacity inside, it is also a good optimization change.

That's what I do, yes. But I noticed the problem because the different screens in our UI fade in and fade out, and the opacity is animated on the full blocks.

I hadn't mentioned it, but the problem also exists for strokes

<Grid
  xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
  xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
  xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
  xmlns:noesis="clr-namespace:NoesisGUIExtensions;assembly=Noesis.GUI.Extensions"
  Background="#FF124C7A">
	<Viewbox>
		<Grid Width="200">
            <StackPanel>
                <Grid Opacity="0.75">
                    <TextBlock
                        Text="Hello world"
                        Foreground="#FFFFFF"
                        HorizontalAlignment="Center" VerticalAlignment="Center"
                        FontWeight="Bold" FontSize="8"
                        noesis:Text.Stroke="Black" noesis:Text.StrokeThickness="4">
                    </TextBlock>
                </Grid>
            </StackPanel>
		</Grid>
	</Viewbox>
</Grid>

image.png (13,665 bytes)   

image.png (13,665 bytes)   

There is a workaround for the issues you are describing with texts (both with italics bounding box, and stroked texts).

The idea is to grow (with padding) the text with its background (using a transparent brush) to account for the italic/stroke:

<Grid
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:noesis="clr-namespace:NoesisGUIExtensions;assembly=Noesis.GUI.Extensions"
Background="#FF124C7A">
  <Viewbox>
    <Grid Width="100">
      <TextBlock
        Text="Outline"
        Background="Transparent"
        Foreground="#FFFFFF"
        HorizontalAlignment="Center"
        FontWeight="Bold" FontSize="16"
        noesis:Text.Stroke="#000000" noesis:Text.StrokeThickness="2" Padding="2" Margin="-2">
        <TextBlock.Effect>
          <DropShadowEffect BlurRadius="0" ShadowDepth="20" Opacity="1" />
        </TextBlock.Effect>
      </TextBlock>
    </Grid>
  </Viewbox>
</Grid>

Could that work with you?

I hadn't thought of doing that, thank you. I think that will do the trick for now :)

DropShadowEffect leaking to other opacity groups fixed in changeset r18067.

I leave 0003553 open because it is a different issue.