Clarify sx prop fallback comments based on compiler analysis

The StyleX compiler's sx→stylex.props transformation works unconditionally;
the hasLocalStyleRef check is an optimization (avoids unnecessary intermediate
rewrite when the compiler would bail to runtime anyway), not a correctness fix.

https://claude.ai/code/session_01EhFJ4W5QixJn7ZLPzMbxPL

Author: claude

src/internal/emit-wrappers/style-merger.ts

@@ -175,8 +175,8 @@ export function emitStyleMerging(args: {
   ) {
     // When useSxProp is enabled, emit sx={expr} instead of {...stylex.props(expr)}
     // Only valid on intrinsic elements (the StyleX babel plugin only processes lowercase tags).
-    // sx requires at least one local styles.* reference for the compiler to recognize;
-    // fall back to stylex.props() when all styles are external (e.g. mixin map lookups).
+    // Prefer sx only when local styles.* references exist; when all styles are external
+    // the compiler bails out of static compilation, so emit stylex.props() directly.
     const sid = emitter.stylesIdentifier;
     const hasLocalRef = styleArgs.some((a) => j([a]).find(j.Identifier, { name: sid }).size() > 0);
     if (emitter.useSxProp && isIntrinsicElement && hasLocalRef) {

src/internal/transform-steps/rewrite-jsx.ts

@@ -793,9 +793,10 @@ export function rewriteJsxStep(ctx: TransformContext): StepResult {
         }
 
         const needsMerge = effectiveClassNameAttr !== null || styleAttr !== null;
-        // sx prop requires at least one local stylex.create() reference so the
-        // StyleX compiler can verify and transform it. When all styles are external
-        // (e.g. only extraStylexPropsArgs mixin lookups), fall back to stylex.props().
+        // Prefer sx prop only when local stylex.create() references exist. When all
+        // styles are external (e.g. only mixin map lookups), the compiler bails out
+        // of static compilation and leaves stylex.props() as a runtime call anyway.
+        // Emitting stylex.props() directly avoids the unnecessary sx→stylex.props rewrite.
         const stylesId = ctx.stylesIdentifier ?? "styles";
         const hasLocalStyleRef = styleArgs.some(
           (arg) => j([arg]).find(j.Identifier, { name: stylesId }).size() > 0,