When freeze=finished is active, ignore iframe.resize.request from the
parent until onContentFrozen runs so pre-freeze measurements cannot
flash wrong thumbnail scales. Add broadcast cleanup for test isolation.

Replace three competing isPreviewLoading effects with one generation
counter so duplicate iframe.resize payloads still clear the loader
after paint instead of waiting for the fallback timeout.

Only hide the in-iframe preparing spinner when review thumbnails use
embed with freeze, keeping generic embed previews on default UI.

Delete computeThumbnailScale and unused content-height defaults so
Frame CSS is the single source of truth for thumbnail scale steps.

Simulate iframe.resize in play functions to assert loaders clear after
measurement, duplicate resize payloads do not stick, and story swaps remount cleanly.

Thumbnail embeds always use freeze=finished for measurement; remove the
ResizeObserver branch and test measurement through onContentFrozen instead.

Move shouldEmbed and shouldAutoplay into embedMode so measurement code
and play-function policy live in separate modules.

…mbnail-content-resize

Keep freeze=finished thumbnails on onContentFrozen-only measurement while
reinstating ResizeObserver-driven sizing for generic embed=true consumers.

Drop progressbar override so the indeterminate spinner keeps correct
aria semantics without missing value attributes.

Use a FrameShell wrapper so aspect-ratio width comes from the cell, not
subgrid row height, and add story coverage for frame-fit-after-resize.

…mbnail-content-resize

Cancel pending debounced RAF on embed resize teardown and assert review
grid frames exist before width checks in story play functions.

Cells now display the loading indicator as soon as they enter the preview
scheduler queue, instead of waiting until the iframe src is assigned.

Ignore identical DISPLAY_REVIEW payloads and only reset expanded
collections when createdAt changes, so opening a story in a new tab
does not collapse the summary the user left open.

… viewport.

Embed previews now report the active viewport name and dimensions alongside content size so CollectionGrid can horizontally scale fixed viewports within the 3:2 frame and clip overflow vertically.

…l-content-resize

Review: Improve review thumbnail scaling and loading UX