Scroll. The 3D tiles must pass under the translucent bar as flat 2D —
never woven over it, never seaming into it. The bar is ordinary sticky HTML with no
inline-3D wiring at all: where the browser occludes by draw order its 2D pixels win over
the weave by themselves; otherwise auto-chrome finds it and excludes it from every tile's
weave. Same page, both paths — the status line says which one is live. Hit
translucency to drop the bar to 45% alpha and watch the wall through it.