fix: address review comments for markerFile

- Merge sidecar map entries in transform.ts when multiple files write
  to the same shared marker path (instead of overwriting)
- Thread markerFile through adapterWithLogging in run.ts so the public
  runTransform API passes it to the per-file transform
- Preserve existing markers in --only regeneration mode by seeding from
  the existing sidecar file
- Extract mergeMarkerContent helper to DRY up the merge logic

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

Author: skovhus

scripts/regenerate-test-case-outputs.mts

@@ -81,6 +81,18 @@ async function listFixtureNames(): Promise<Array<{ name: string; ext: string }>>
     .sort((a, b) => a.name.localeCompare(b.name));
 }
 
+/** Merge marker declarations from `incoming` into `base`, appending only new exports. */
+function mergeMarkerContent(base: string, incoming: string): string {
+  const markerLineRe = /^export const \w+ = stylex\.defineMarker\(\);$/gm;
+  const newMarkers = [...incoming.matchAll(markerLineRe)].map((m) => m[0]);
+  const markersToAdd = newMarkers.filter((line) => !base.includes(line));
+  if (markersToAdd.length === 0) {
+    return base;
+  }
+  const trailingNewline = base.endsWith("\n") ? "" : "\n";
+  return base + trailingNewline + markersToAdd.join("\n") + "\n";
+}
+
 // Accumulate all sidecar files across fixtures for merging (shared marker files)
 const allSidecarFiles = new Map<string, string>();
 
@@ -113,21 +125,7 @@ async function updateFixture(name: string, ext: string) {
   // Accumulate sidecar files for merging after all fixtures are processed
   for (const [sidecarPath, content] of sidecarFiles) {
     const existing = allSidecarFiles.get(sidecarPath);
-    if (existing) {
-      // Merge: extract new marker lines and append any that don't already exist
-      const markerLineRe = /^export const \w+ = stylex\.defineMarker\(\);$/gm;
-      const newMarkers = [...content.matchAll(markerLineRe)].map((m) => m[0]);
-      const markersToAdd = newMarkers.filter((line) => !existing.includes(line));
-      if (markersToAdd.length > 0) {
-        const trailingNewline = existing.endsWith("\n") ? "" : "\n";
-        allSidecarFiles.set(
-          sidecarPath,
-          existing + trailingNewline + markersToAdd.join("\n") + "\n",
-        );
-      }
-    } else {
-      allSidecarFiles.set(sidecarPath, content);
-    }
+    allSidecarFiles.set(sidecarPath, existing ? mergeMarkerContent(existing, content) : content);
   }
 
   return outputPath;
@@ -159,7 +157,17 @@ for (const { name, ext } of targetFixtures) {
   await updateFixture(name, ext);
 }
 
-// Write accumulated sidecar files (merged across all fixtures)
+// Write accumulated sidecar files (merged across all fixtures).
+// When running --only, seed from existing sidecar files so markers from untouched fixtures are preserved.
 for (const [sidecarPath, content] of allSidecarFiles) {
-  await writeFile(sidecarPath, content, "utf-8");
+  let merged = content;
+  if (only) {
+    try {
+      const existing = await readFile(sidecarPath, "utf-8");
+      merged = mergeMarkerContent(content, existing);
+    } catch {
+      // File doesn't exist yet, write fresh content
+    }
+  }
+  await writeFile(sidecarPath, merged, "utf-8");
 }

src/run.ts

@@ -428,6 +428,7 @@ export async function runTransform(options: RunTransformOptions): Promise<RunTra
       ? resolveBaseComponentWithLogging
       : undefined,
     resolveThemeCall: resolvedAdapter.resolveThemeCall,
+    markerFile: resolvedAdapter.markerFile,
   };
 
   // Path to the transform module.

src/transform.ts

@@ -70,7 +70,22 @@ export default function transform(file: FileInfo, api: API, options: Options): s
         const sidecarPath =
           result.sidecarFilePath ??
           join(dirname(file.path), `${basename(file.path).replace(/\.\w+$/, "")}.stylex.ts`);
-        sidecarFiles.set(sidecarPath, result.sidecarContent);
+        // Merge with existing content when multiple files write to the same sidecar path
+        const existing = sidecarFiles.get(sidecarPath);
+        if (existing) {
+          const markerLineRe = /^export const \w+ = stylex\.defineMarker\(\);$/gm;
+          const newMarkers = [...result.sidecarContent.matchAll(markerLineRe)].map((m) => m[0]);
+          const markersToAdd = newMarkers.filter((line) => !existing.includes(line));
+          if (markersToAdd.length > 0) {
+            const trailingNewline = existing.endsWith("\n") ? "" : "\n";
+            sidecarFiles.set(
+              sidecarPath,
+              existing + trailingNewline + markersToAdd.join("\n") + "\n",
+            );
+          }
+        } else {
+          sidecarFiles.set(sidecarPath, result.sidecarContent);
+        }
       }
     }
 

test-cases/markers.stylex.ts

@@ -1,11 +1,11 @@
 import * as stylex from "@stylexjs/stylex";
 
+export const ThingMarker = stylex.defineMarker();
+export const ThingThemedMarker = stylex.defineMarker();
+export const RowMarker = stylex.defineMarker();
 export const WrapperMarker = stylex.defineMarker();
 export const LinkMarker = stylex.defineMarker();
 export const StyledCollapseButtonMarker = stylex.defineMarker();
 export const IconButtonMarker = stylex.defineMarker();
 export const CrossFileLinkMarker = stylex.defineMarker();
-export const ThingMarker = stylex.defineMarker();
 export const IconMarker = stylex.defineMarker();
-export const ThingThemedMarker = stylex.defineMarker();
-export const RowMarker = stylex.defineMarker();