Fix UI extension dev bundle path

[devisscher]

WHY are these changes introduced?

Fixes https://github.com/shop/issues-discounts/issues/2117

shopify app dev regressed for generic ui_extension extensions whose bundle step writes assets under a configured bundle folder. The generated bundle manifest points at the real script path, for example dist/<handle>.js, but draft upload was still reading extension.getOutputPathForDirectory(bundlePath), which resolves to <handle>.js for generic UI extensions.

WHAT is this pull request doing?

• Updates updateExtensionDraft() to resolve generic ui_extension serialized scripts from the bundle manifest.json main path when available.
• Keeps the existing output-path fallback when the manifest is missing, malformed, lacks a usable main, or resolves outside the bundle directory.
• Gates the manifest-aware behavior to generic ui_extension, preserving legacy checkout_ui_extension and web_pixel_extension behavior.
• Adds focused unit coverage for manifest path usage, missing-manifest fallback, and unchanged checkout UI extension serialization.

How to test your changes?

Targeted test command:

pnpm --filter @shopify/app vitest run src/cli/services/dev/update-extension.test.ts

Local note: I attempted this command, but dependencies are not installed in this checkout (vitest: command not found; node_modules missing), so no tests executed locally.

Manual verification path:

1. In an app with a generic ui_extension targeting checkout and a generated bundle manifest like { "purchase.checkout.block.render": { "main": "dist/<handle>.js" } }, run shopify app dev.
2. Confirm draft upload reads <dev-bundle>/<uid>/dist/<handle>.js for serialized_script instead of <dev-bundle>/<uid>/<handle>.js.
3. Confirm legacy checkout_ui_extension and web_pixel_extension draft uploads still read their normal output paths.

Checklist

• I've considered possible cross-platform impacts (Mac, Linux, Windows)
• I've considered possible documentation changes
• I've considered analytics changes to measure impact
• The change is user-facing — I've identified the correct bump type (patch for bug fixes · minor for new features · major for breaking changes) and added a changeset with pnpm changeset add

[Copilot]

Pull request overview

Fixes a regression in shopify app dev draft uploads for generic ui_extension extensions where the bundle output is written under a configured subfolder (e.g. dist/). The change updates how serialized_script is located so it matches the actual bundle output indicated by the generated manifest.json, while preserving legacy behavior for other extension types.

Changes:

• Resolve ui_extension serialized script path from manifest.json (main) when available and safely within the bundle directory.
• Keep a fallback to extension.getOutputPathForDirectory(bundlePath) when the manifest is missing/invalid/unsafe.
• Add unit tests covering manifest usage for generic ui_extension, missing-manifest fallback, and unchanged behavior for checkout_ui_extension.

Reviewed changes

Copilot reviewed 3 out of 3 changed files in this pull request and generated no comments.

File	Description
packages/app/src/cli/services/dev/update-extension.ts	Adds manifest-aware path resolution for ui_extension draft uploads with safety checks and fallback behavior.
packages/app/src/cli/services/dev/update-extension.test.ts	Adds focused tests validating manifest-based resolution, fallback, and legacy behavior preservation.
.changeset/fix-ui-extension-dev-bundle-path.md	Adds a patch changeset documenting the fix for generic UI extension dev draft uploads.

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