#1531 open-design-landing-deck can render cropped with black area on the right in artifact preview

Summary

open-design-landing-deck can render incorrectly inside the Open Design artifact preview.

In the stable desktop app, the generated HTML opens, but the preview does not show the deck cleanly: the page can appear cropped / not fully visible, with a black area on the right side.

Why this looks like a host preview bug

This does not look like a simple malformed artifact.

open-design-landing-deck is not a conventional fixed-canvas deck. It is a full-viewport horizontal swipe deck that relies on:

position: fixed
100vw / 100vh
a horizontally translated flex track
slide movement via transform: translateX(-idx * 100vw)

The template explicitly assumes a browser-style viewport.

Relevant implementation details

The template itself sets up a full-screen horizontal track, for example in design-templates/open-design-landing-deck/example.html:

#deck { position: fixed; inset: 0; height: 100vh; display: flex; ... }
.slide { width: 100vw; height: 100vh; flex: 0 0 100vw; ... }
runtime script: deck.style.width = (total * 100) + 'vw'
runtime script: deck.style.transform = 'translateX(' + (-idx * 100) + 'vw)'

Open Design preview, however, renders the artifact inside a sandboxed iframe and wraps it in a preview viewport / scale shell in apps/web/src/components/FileViewer.tsx.

The host also injects extra deck behavior from apps/web/src/runtime/srcdoc.ts.

That means this template is effectively running inside:

a sandboxed iframe
a host-managed preview viewport
an outer scaling shell

This is likely where the incompatibility appears.

Additional evidence

The template already contains explicit comments about cooperating with Open Design's host-driven navigation, which suggests this surface is known to be special:

it installs its own od:slide message handler
it uses a MutationObserver to stay in sync with host bridge class changes

So this is not just a generic HTML deck; it is already trying to compensate for host preview behavior.

Expected behavior

The generated open-design-landing-deck artifact should render fully within the preview canvas without cropping or a black area on the right.

Actual behavior

The HTML opens, but the preview can appear partially visible / cropped, with a black region on the right side.

Likely root cause

A compatibility issue between:

the template's full-viewport assumptions (fixed, 100vw, 100vh, horizontal translateX track)
and Open Design's iframe + preview scaling shell + injected deck bridge

Suggested investigation areas

apps/web/src/components/FileViewer.tsx
- preview viewport sizing / scale shell
apps/web/src/runtime/srcdoc.ts
- injected deck bridge behavior
design-templates/open-design-landing-deck/example.html
- dependency on window.innerWidth, 100vw, 100vh, fixed positioning

Nice-to-have regression coverage

Add a preview regression case for a full-viewport horizontal deck template like open-design-landing-deck to ensure:

slide 1 is fully visible
no black gutter appears on the right
host-driven navigation keeps the translated track aligned with the preview viewport