UV Offset Effect Material in RetainerBox Causes Visual Artifacts in Editor Preview

When a UV Offset is applied in a RetainerBox effect material, visual artifacts appear only in the editor — not at runtime — if a widget with a dotted preview outline (like CanvasPanel) is placed directly under the RetainerBox.

This artifact becomes more noticeable when the texture sampler’s wrap mode is set to Clamp. A RenderDoc capture confirms that the dotted preview lines are captured in the RetainerBox’s offscreen render target and then get dragged visually due to clamped UV sampling.

In-game rendering works as expected. The issue appears to be tied to the editor’s preview system and how it renders widget outlines during offscreen rendering.

1. Create a blank project.
2. Create a new Widget Blueprint.
3. Add a RetainerBox, then place a CanvasPanel as its child.
4. Create a new Material.
5. Add a TextureSampleParameter2D node with Parameter Name = "Texture" and Sampler Source = Shared: Clamp.
6. Apply a UV offset to the sampled texture (e.g., add (0.5, 0.5) to the UVs).
7. In the Widget Blueprint Editor, set the material as the RetainerBox’s Effect Material.
8. Assign a texture to the "Texture" parameter, and ensure Show Effects in Designer is checked.

You see a weird striped pattern on the screen.

You can use the attached repro project. Download and open it then open the Widget Blueprint from the content browser.