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Because reasons, which shadow map the shader selects for each fragment is chosen in a really inefficient way. If allow shadow map overlap = true is made mandatory, we can do what every other engine ever does and select based on the planar distance from the eye point and just pass the threshold distances to the shader instead of several sets of matrices and checking whether the fragment position in various spaces is within a certain region.

I thought I'd put this on the tracker a while ago, but apparently it's just been mentioned in the comments of other issues a lot of times.