"step0 target  isIntersecting=true isVisible=true  (target IS fully covered by opaque drop-shadow pixels)"
"step0 control isIntersecting=true isVisible=true  (control unobstructed -> should be true)"
"step0 elementFromPoint(100,20) -> #target  (user click on the black overlay reaches the target)"
"step1 target  isVisible=true  (filter removed -> truly visible)"
"step1 control isVisible=false  (plain-div occluder -> should be false)"
"VULN-CONFIRMED: IO-V2 reported isVisible=true while target was fully occluded by an inner-<svg> drop-shadow; the same observer correctly reported isVisible=false for a plain-div occluder."