Some HTML emails set height:100% on a full-bleed wrapper table/div rather
than html/body. With the viewer's body{overflow:hidden}, this collapses
documentElement.scrollHeight to the iframe's 150px default, so the
scrollHeight-based auto-resize locks the iframe short and the body renders
blank below the fold. A Box.co.il verification email rendered as a
logo-only 150px strip.
- Neutralise height:100% on any element so the body grows to its content.
- Measure max(documentElement, body).scrollHeight, and re-measure on a
fixed cadence over a short settle window so a late reflow (height:100%
wrapper, or images that resize after onload) is caught even when no
ResizeObserver/image event fires.
