Fading a neighbouring 3D object means every part of it, and right after the focused one

uniform vec2 uPanelFade;    // screen shares where the fade starts and where it is complete
uniform float uPanelAmount; // 0..1: how much this shelf gives way, eased by the room
uniform vec2 uViewport;     // the render target's size in pixels
float panelKeep(float x) {
  return uPanelFade.x < uPanelFade.y ? 1.0 - smoothstep(uPanelFade.x, uPanelFade.y, x)
                                     : smoothstep(uPanelFade.y, uPanelFade.x, x);  // either side, for right to left
}
// In every shader that draws part of a shelf: the shoe, the slab, the brackets, and the wall's shadows of it.
float keep = mix(1.0, panelKeep(gl_FragCoord.x / uViewport.x), uPanelAmount);
outColor *= keep;   // premultiplied output, so scaling all four channels fades it