• check for invalid index values in decoded sourcemaps of existing samples
• find and fix why existing sourcemap tests did not detect this ( mappings should be traced back more strictly)
• find and fix underlying issue

Before submitting the PR, please make sure you do the following

• It's really useful if your PR references an issue where it is discussed ahead of time. In many cases, features are absent for a reason. For large changes, please create an RFC:https://github.com/sveltejs/rfcs
• Prefix your PR title withfeat:,fix:,chore:, ordocs:.
• This message body should clearly illustrate what problems it solves.
• Ideally, include a test that fails without this PR but passes with it.
• If this PR changes code withinpackages/svelte/src, add a changeset (npx changeset).

Tests and linting

• Run the tests withpnpm test and lint the project withpnpm lint