The frame was a hardcoded 280px unless the model guessed a height attribute
— tall pages clipped (the flip-clock demo cut its last digit), short ones
floated in dead space. The opaque-origin sandbox means the parent can't
measure the document, but we own the srcdoc string: a tiny injected script
observes the document with ResizeObserver and posts its scrollHeight up via
postMessage, and the frame tracks it live within the 120-1200 clamp.
Reports are validated before they can move layout — per-mount random token
(two previews in one transcript, or a hostile page inventing messages,
can't move each other's frames), finite-number check, clamp. A 4px
tolerance stops vh-sized pages (which measure exactly what they're given)
from oscillating; an explicit height attribute still opts out of
auto-sizing entirely.
Brooklyn Nicholson
5c4f1e744c