A 3D shadow map is a 2D depth image; flatten the world by one dimension and it becomes a 1D strip of depths. Everything that goes wrong in 3D goes wrong here too, visibly.
Drag in the upper area to move the light. Hover the floor to probe one pixel's depth comparison.
What to notice. At bias 0 the floor breaks into stripes — half of every texel's footprint is farther from the light than the one depth that texel stored. That is shadow acne. Crank the bias and the stripes go, but the shadow under the left box detaches from its base: peter-panning. Lower the resolution and the shadow edge turns into staircase steps that no amount of bias will fix.
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