Skip unchanged stories with --only-changed

On a large Storybook most pull requests touch a handful of components. Re-rendering all 1,000 stories every time is slow, and on any per-snapshot tool it is expensive. --only-changed renders just the stories your change can affect and carries the unchanged ones forward from their baseline.

--only-changed walks your pull request's file changes against the module dependency graph and selects every story that imports a changed file, directly or transitively. Everything else is carried forward: its existing baseline is reused as this build's result, with no render and no diff.

• No preview-stats.json in the build - you did not pass --stats-json, so there is no graph to trace.
• A change to a file many stories import - a global stylesheet, a design-token file, the Storybook preview.
• A lockfile or dependency change, which can shift anything.
• The first build on a branch, which has nothing to carry forward yet.

In each of those cases re-rendering everything is the correct, safe answer, so it does.

No. --only-changed is a Storybook feature - it needs the module dependency graph to trace which stories a change affects, and a Playwright archive has no such graph. Archive builds always render what your tests captured. To keep those fast and cheap, make the captures deterministic instead - see Fix flaky visual tests.

A carried-forward story is one whose inputs your change did not touch, so its pixels should not have moved. The guarantee is only as good as the change detection, though, so treat any surprise as a signal: if you ever see a carried-forward story that clearly should have rendered, or a diff you cannot attribute to your own change, investigate the baseline rather than accepting it.