Some emails (Outlook / templated HTML) set `html, body { height: 100% }` in
their own <style>. Combined with the viewer srcDoc's `overflow: hidden` and the
scrollHeight-based iframe auto-resize, the measured height collapses to the
iframe's initial size, so everything below the first screenful (often just the
header/logo) is clipped and the rest of the message is invisible.
Force `height: auto !important` on html/body in the rendered srcDoc so the
document grows to its real content height before scrollHeight is measured.
