fix: resolve sources inside the sourceRoot directory on filesystem paths

.changeset/resolve-source-root-directory.md

@@ -0,0 +1,5 @@
+---
+"rollup-plugin-sourcemaps2": patch
+---
+
+Resolve sources inside the map's `sourceRoot` directory on filesystem paths. The trailing slash of the root was stripped, so `sourceRoot: "src"` with `sources: ["index.ts"]` resolved to `index.ts` next to the map instead of `src/index.ts`, and the map lost its `sourcesContent`.

src/__tests__/index.test.ts

@@ -8,7 +8,7 @@ import { type PluginContext, rollup } from 'rollup';
 import { describe, expect, it, test, vi } from 'vitest';
 
 import sourcemaps, { type SourcemapsPluginOptions } from '..';
-import { resolveSourceMap } from '../source-map-resolve';
+import { resolveSourceMap, resolveSources } from '../source-map-resolve';
 
 const inputPath = path.join(__dirname, '../index.ts');
 const inputText = fs.readFileSync(inputPath, 'utf8');
@@ -332,6 +332,32 @@ it('finds last source map file definition', async () => {
   expect(map?.url).toEqual('/dev/null/index.js.map');
 });
 
+it('resolves sources inside the sourceRoot directory', async () => {
+  const mapUrl = path.join(__dirname, 'dist', 'index.js.map');
+  const requested: string[] = [];
+
+  const { sourcesResolved, sourcesContent } = await resolveSources(
+    {
+      version: 3,
+      file: 'index.js',
+      sourceRoot: 'src',
+      sources: ['index.ts'],
+      names: [],
+      mappings: '',
+    },
+    mapUrl,
+    async source => {
+      requested.push(source);
+      return 'source text';
+    },
+  );
+
+  const expected = path.join(__dirname, 'dist', 'src', 'index.ts');
+  expect(sourcesResolved).toStrictEqual([expected]);
+  expect(requested).toStrictEqual([expected]);
+  expect(sourcesContent).toStrictEqual(['source text']);
+});
+
 it('skips directories even when the filesystem lets them be read', async () => {
   const dir = fs.mkdtempSync(path.join(os.tmpdir(), 'sourcemaps2-'));
   // Mimic macOS devfs, where read() on the /dev directory succeeds and returns raw bytes.

src/source-map-resolve.ts

@@ -40,7 +40,9 @@ function resolveUrl(...args: string[]): string {
       continue;
     }
 
-    resolved = path.resolve(path.dirname(resolved), nextUrl);
+    // A trailing slash marks a directory (sourceRoot), so the next segment resolves inside it
+    const base = resolved.endsWith('/') ? resolved : path.dirname(resolved);
+    resolved = path.resolve(base, nextUrl) + (nextUrl.endsWith('/') ? '/' : '');
   }
 
   return resolved;