We have a longstanding bug where a thin line is drawn on the right and bottom edges of a directly composited image that is marked as opaque. To reproduce, change GraphicsLayer::SetContentsToImage and add the following at the bottom:
  image_layer_->SetContentsOpaque(true);
  SetContentsTo(image_layer_.get(), /*prevent_contents_opaque_changes=*/true);
Then open the attached testcase.

This artifact comes from RasterSource::ClearForOpaqueRaster which draws extra to workaround clamping bugs in the gpu image drawer (shader?).
 

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