Ensure PostCSS outputs source maps for Sass sources

We transform Sass sources (Autoprefixer only) but didn’t source map the changes

shared/tasks/build/package.test.mjs

@@ -70,6 +70,12 @@ describe('package/', () => {
         return output
       }))
 
+      // Add Autoprefixer prefixes to all source '*.scss' files
+      .flatMap(mapPathTo(['**/*.scss'], ({ dir: requirePath, name }) => [
+        join(requirePath, `${name}.scss`),
+        join(requirePath, `${name}.scss.map`) // with source map
+      ]))
+
       // Replaces source component '*.yaml' with:
       // - `fixtures.json` fixtures for tests
       // - `macro-options.json` component options

shared/tasks/styles.mjs

@@ -43,10 +43,25 @@ export async function compileStylesheet ([modulePath, { srcPath, destPath, fileP
   let css
   let map
 
-  // Configure PostCSS
+  /**
+   * Configure PostCSS
+   *
+   * @type {import('postcss').ProcessOptions}
+   */
   const options = {
     from: moduleSrcPath,
-    to: moduleDestPath
+    to: moduleDestPath,
+
+    /**
+     * Always generate source maps for either:
+     *
+     * 1. PostCSS on Sass compiler result
+     * 2. PostCSS on Sass sources (Autoprefixer only)
+     */
+    map: {
+      annotation: true,
+      inline: false
+    }
   }
 
   // Compile Sass to CSS
@@ -79,10 +94,8 @@ export async function compileStylesheet ([modulePath, { srcPath, destPath, fileP
     }))
 
     // Pass source maps to PostCSS
-    options.map = {
-      annotation: true,
-      inline: false,
-      prev: map
+    if (typeof options.map === 'object') {
+      options.map.prev = map
     }
   }