[Fiber] render boundary in fallback if it contains a new stylesheet during sync update

When we implemented Suspensey CSS we had a heuristic that if the update was sync we would ignore the loading states of any new stylesheets and just do the commit. But for a stylesheet capability to be useful it needs to reliably prevent FOUC and since the stylesheet api is opt-in through precedence we don't have to maintain backaward compat (old stylesheets do not block commit but then nobody really renders them because of FOUC anyway)

This update modifies the logic to put a boundary back into fallback if a sync update would lead to a stylesheet commiting before it loaded.

Author: gnoff

Diff for: packages/react-dom-bindings/src/client/ReactFiberConfigDOM.js

@@ -3154,7 +3154,6 @@ export function mayResourceSuspendCommit(resource: Resource): boolean {
 }
 
 export function preloadInstance(type: Type, props: Props): boolean {
-  // Return true to indicate it's already loaded
   return true;
 }
 
@@ -3163,10 +3162,11 @@ export function preloadResource(resource: Resource): boolean {
     resource.type === 'stylesheet' &&
     (resource.state.loading & Settled) === NotLoaded
   ) {
-    // we have not finished loading the underlying stylesheet yet.
+    // Return false to indicate this resource should suspend
     return false;
   }
-  // Return true to indicate it's already loaded
+
+  // Return true to indicate this resource should not suspend
   return true;
 }