When using screen aligned UVs they don't seem to account for letterboxing. When the aspect ratio is changed the UVs scale based off the top left of the window or screen and not the viewable area.
The target texture displayed on the mesh seems to scale the aspect ratio but isn't taking the letterbox (black bars) into account and is using the top left of the window as it's 0,0, coordinate instead of the actual view area.
The texture displayed takes the letterbox into account when scaling the UVs
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