Translucent materials in the PostMotionBlur pass can cause a flickering viewport artefact when using TAA

There appears to be a bug relating to rendering multiple windows that can trigger under unknown circumstances. It has been reported on rectangle lights in the forums and on PostMotionBlur materials on the UDN. It has also rarely been seen occurring to reflections when disabling RayTraced Shadows with no stable reproduction.

This report focuses on the PostMotionBlur reports as a semi-stable reproduction has been reported.

From the repro, the following has been noted: limiting frame rate with t.MaxFPS will alleviate the issue if the limit is being consistently hit. This oddly conflicts with reports that VSync can increase the likelihood of the issue occurring. Uncapping the framerate leads to the highest rate of bad frames v. good by virtue of the frame time seemingly to affect the incident rate of bad frames.

Once the issue is triggered, it will only occur when a tooltip or a second editor window is being rendered and the PostMotionBlur material is on screen.

OneThread prevents the issue from arising, but it is not clear if this is due to the large increase in gamethread time.

PIX captures do not show the issue. All buffers used in the rendering process appear to be correct, including the problem area "ComposeTranslucencyToNewSceneColor(AfterMotionBlur)". When the issue is present, the output buffer – PostMotionBlurTranslucency.SceneColor – is the first to show the issue, with VisualizeTexture showing all inputs correct, but the output incorrect.

Common to all the viewport flicker artefacts is the general appearance that the ViewRect for texture resource being sampled, in this case Translucency.AfterMotionBlur.Color, is incorrectly sized. PIX reports the size correctly in the name of the event.

As a note: a common thread among internal testing, the UDN, and the Unreal forums is the use of NVIDIA hardware; however, there doesn't appear to be much AMD-based testing of the issue either.

Create a translucent material and place it in the level.
Open the material and move the editor window to second monitor such that the engine is now open on both monitors.
Switch the material to the PostMotionBlur and save.
If nothing has gone wrong, revert the pass to BeforeDOF, save all, and restart the editor. Repeat until the bug presents itself.

Expected and most common case: Material renders differently, as expected.
Unexpected: Screenspace location of the materials flickers as if the view size is incorrect for the pass.
Other results: The entire viewport flickers as above, not just the material. This unexpected result appears to occur most commonly with PIX attached, as opposed to the above without.

This reproduction is not overly consistent, but it does succeed often enough to garner some insight into what is happening.
Do note that PIX, nsights, and DumpGPU will not show the issue. Neither will running with -onethread.

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