The public share page rooted on `.desktop-title-bar-page`, which sets only
`min-height: 100vh`. Without a definite height, `h-full` on every descendant of
`<main>` resolved to `auto` -> 0. `HtmlPreview` gates its sandboxed iframe on a
measured non-zero container, so it silently never mounted and the page rendered
a blank area under the header. Other read-only branches survived because their
content has intrinsic height.
Give the public page root a definite height, and make the read-only preview
chain flex-based so it fills its parent instead of depending on an ancestor's
definite height. `text-editor`'s preview pane becomes a flex column for the same
reason -- it is `HtmlPreview`'s other parent.