"Screen scissor" is the minimum size of all render targets, and is set when any render target is bound on NVN or OpenGL. Since it works on all active texture's real sizes, it is therefore more reliable than viewport 0's width, and is actually set before clear.
This fixes a regression with Hyrule Warriors: Age Of Calamity's cubemaps, which did not set viewport dimensions before clear. This resulted in attempting to create a cubemap with rectangular sides, which is logically and physically impossible. (also it just fails)
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