fix: preserve trailingSlash config when error boundary is above page node

[danielalanbates]

Summary

Fixes #13516

When an error occurs during client-side rendering and the branch is sliced to the nearest error boundary, the trailingSlash config from deeper nodes (e.g. the page's +page.js) is lost because those nodes are removed from the branch. This causes get_navigation_result_from_branch to fall back to the default 'never' setting, stripping the trailing slash from the URL even when trailingSlash = 'always' is configured on the page.

Changes

• packages/kit/src/runtime/client/client.js: Added a trailing_slash parameter to get_navigation_result_from_branch that serves as a fallback when the (sliced) branch contains no nodes with a slash property. At both call sites where the branch is sliced for error handling, the trailingSlash value is extracted from the full branch before slicing and passed through.

• Test: Added a test case that navigates to a page with trailingSlash = 'always' that throws an error, verifying the URL retains the trailing slash when the error boundary (defined at a higher level) catches the error.

How it works

The root cause is that branch.slice(0, error_load.idx).concat(error_load.node) removes the page node that contained the trailingSlash config. The fix preserves this config by:

1. Scanning the full branch for slash before slicing
2. Passing the found value as trailing_slash to get_navigation_result_from_branch
3. Using it as the initial value of slash (instead of 'never'), which can still be overridden by any slash property found in the sliced branch

This PR was created with the assistance of Claude Opus 4.6 by Anthropic. Happy to make any adjustments!

[changeset-bot]

🦋 Changeset detected

Latest commit: b30c5bd

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 1 package

Name	Type
@sveltejs/kit	Patch

Not sure what this means? Click here to learn what changesets are.

Click here if you're a maintainer who wants to add another changeset to this PR

[vercel]

JSDoc type definition for get_navigation_result_from_branch is missing the trailing_slash parameter, causing TypeScript errors

[benmccann]

It looks like you need to run pnpm format and fix some failing tests. E.g.:

    1) [chromium-dev] › test/cross-platform/client.test.js:1158:2 › Routing › trailing slash is preserved when error boundary is above page config 
  
      Error: expect(received).toBe(expected) // Object.is equality
  
      Expected: "/routing/trailing-slash/error-boundary/"
      Received: "/routing/trailing-slash/error-boundary"
  
        1160 | 		// The page throws an error, which triggers the error boundary.
        1161 | 		// The URL should still have the trailing slash from the page's trailingSlash='always' config.
      > 1162 | 		expect(new URL(page.url()).pathname).toBe('/routing/trailing-slash/error-boundary/');
             | 		                                     ^
        1163 | 	});
        1164 | });
        1165 |
          at /home/runner/work/kit/kit/packages/kit/test/apps/basics/test/cross-platform/client.test.js:1162:40

[teemingc]

It's probably better to create a util for this so we don't repeat it so many times. It's currently repeated 3 times in the module.

Maybe something like:

/**
 * @param {(import('./types.js').BranchNode | undefined)[]} branch
 * @param {URL} url
 * @returns {import('types').TrailingSlash}
 */
function get_trailing_slash(branch, url) {
	// if `paths.base === '/a/b/c`, then the root route is always `/a/b/c/`, regardless of
	// the `trailingSlash` route option, so that relative paths to JS and CSS work
	if (base && (url.pathname === base || url.pathname === base + '/')) {
		return 'always';
	}

	return branch.reduce((value, node) => {
		return node?.slash ?? value;
	}, /** @type {import('types').TrailingSlash} */ ('never'));
}

[teemingc]

This is almost the right solution but we need one more change. The trailing slash info for the node isn't returned when the load function fails. Ideally, we'd still keep track of it even if the load function fails. We need to make changes so that we get

kit/packages/kit/src/runtime/client/client.js

Line 890 in fca32df

slash: node.universal?.trailingSlash ?? server_data_node?.slash

even if the load throws

[teemingc]

We probably want to compute the trailing slash option here too so that it respects the root layout's trailing slash option