Problems when applying shaders to the back buffer

Development Rendering
1

I’m trying to apply a shader directly to the back buffer from C++ in a subsystem, so that I can apply processing to the texture of the scene and UI rendered together.

I’m currently binding to FSlateRenderer::OnBackBufferReadyToPresent, which is the last step before the render target is presented to the GPU as far as I can tell.

It’s almost working, I am just trying to do the most basic thing and run a pixel shader that just returns a solid color for now. And that color is appearing, but it is EXTREMELY glitchy. I’m not even sure where to begin debugging it.
I wish I could explain this better, but what I’m seeing is that in the top-right quadrant of the screen, a bunch of solid colored triangles are flickering in and out.

Here is the code:

void URenderSubsystem::BeginPIE( const bool bIsSimulating )
{
    FSlateApplication::Get().GetRenderer()->OnBackBufferReadyToPresent().AddUObject( this, &URenderSubsystem::OnBackBufferReady_RenderThread );
}

void URenderSubsystem::OnBackBufferReady_RenderThread( SWindow& SlateWindow, const FTexture2DRHIRef& BackBuffer )
{
    FGlobalShaderParams Params;
    Params.Color = FVector4( 1, 0, 0, 1 );
    FRHICommandListImmediate& RHICmdList = GRHICommandList.GetImmediateCommandList();
    const FName TextureRenderTargetName = BackBuffer->GetName();
    FMyGlobalShader::DrawColorToRenderTarget_RenderThread( RHICmdList, Params, TextureRenderTargetName, BackBuffer, ERHIFeatureLevel::SM5 );
 }

// …

void FMyGlobalShader::DrawColorToRenderTarget_RenderThread(
    FRHICommandListImmediate& RHICmdList,
    const FGlobalShaderParams ShaderParams,
    const FName& TextureRenderTargetName,
    FTexture2DRHIRef OutTexture,
    ERHIFeatureLevel::Type FeatureLevel)
{
    check(IsInRenderingThread());

    RHICmdList.TransitionResource(EResourceTransitionAccess::EWritable, OutTexture);

    FRHIRenderPassInfo RPInfo(OutTexture, ERenderTargetActions::DontLoad_Store, OutTexture);
    RHICmdList.BeginRenderPass(RPInfo, TEXT("DrawColor"));
    {
        FIntPoint Res(OutTexture->GetSizeX(), OutTexture->GetSizeY());

        RHICmdList.SetViewport(0, 0, 0.f, Res.X, Res.Y, 1.f);

        FGlobalShaderMap* GlobalShaderMap = GetGlobalShaderMap(FeatureLevel);
        TShaderMapRef< FGlobalShaderVS > VertexShader(GlobalShaderMap);
        TShaderMapRef< FGlobalShaderPS > PixelShader(GlobalShaderMap);

        FGraphicsPipelineStateInitializer Init;
        RHICmdList.ApplyCachedRenderTargets(Init);
        Init.DepthStencilState = TStaticDepthStencilState<false, CF_Always>::GetRHI();
        Init.BlendState = TStaticBlendState<>::GetRHI();
        Init.RasterizerState = TStaticRasterizerState<>::GetRHI();
        Init.PrimitiveType = PT_TriangleList;
        Init.BoundShaderState.VertexDeclarationRHI = GetVertexDeclarationFVector4();
        Init.BoundShaderState.VertexShaderRHI = VertexShader.GetVertexShader();
        Init.BoundShaderState.PixelShaderRHI = PixelShader.GetPixelShader();
        SetGraphicsPipelineState(RHICmdList, Init);

        VertexShader->SetParameters(RHICmdList, VertexShader.GetVertexShader(), ShaderParams);
        PixelShader->SetParameters(RHICmdList, PixelShader.GetPixelShader(), ShaderParams);

        uint32 PrimitiveCount = kGridSubdivisionX * kGridSubdivisionY * 2;
        RHICmdList.DrawPrimitive( 0, PrimitiveCount, 1 );
    }
    RHICmdList.EndRenderPass();
}

Hopefully this makes sense. Thanks!