A clear box drawn without a depth buffer must be cut at the plane of what stands inside it

// box.ranges[name]: [first vertex, count] of one face of a unit cube; the vertex shader places it by uOrigin + aPos * uScale.
// uBoxMin and uBoxMax are set to the whole slab first. World y points down; the camera is always in front.
// before: the whole slab after the shoe, so the back half of its top covered the shoe
function drawSlab(gl, u, box, min, max, cam, shoeZ, drawShoe) {
  const face = (name, z0 = min[2], z1 = max[2]) => {
    gl.uniform3f(u.uOrigin, min[0], min[1], z0);
    gl.uniform3f(u.uScale, max[0] - min[0], max[1] - min[1], z1 - z0);
    gl.drawArrays(gl.TRIANGLES, ...box.ranges[name]);
  };
  const below = cam.y > max[1]; // the eye is under the slab
  if (!below) face("bottom");
  face("top", min[2], shoeZ); // the part of the top behind the shoe
  if (cam.x > min[0]) face("left");
  if (cam.x < max[0]) face("right");
  drawShoe(); // draws with its own program, then binds the slab's again
  face("top", shoeZ, max[2]); // the part in front of it
  if (below) face("bottom");
  if (cam.x < min[0]) face("left");
  if (cam.x > max[0]) face("right");
  face("front");
}

// The rim from the whole box's edges, in world units, so the cut at the shoe's plane draws no line.
vec3 dd = min(vWorld - uBoxMin, uBoxMax - vWorld);
float edge = abs(N.z) > 0.5 ? min(dd.x, dd.y) : (abs(N.y) > 0.5 ? min(dd.x, dd.z) : min(dd.y, dd.z));