feat(dom, css): add MaybeCommitFirstPaintStyleSync to finalize first-paint style updates

- Introduced `ExecutingContext::MaybeCommitFirstPaintStyleSync` to clear first-paint style sync barriers after a synchronous style update.
- Ensured deferred UI commands are flushed post-update for accurate `getComputedStyle()` results.
- Updated `Document::UpdateStyle()` to call the new method.
- Adjusted and normalized integration tests for first-paint barrier scenarios.

Author: CGQAQ

bridge/core/dom/document.cc

@@ -684,6 +684,7 @@ void Document::UpdateStyleForThisDocument() {
   style_engine.InvalidateEnvDependentStylesIfNeeded();
   UpdateStyleInvalidationIfNeeded();
   UpdateStyle();
+  context->MaybeCommitFirstPaintStyleSync();
 
   auto duration_ms =
       std::chrono::duration_cast<std::chrono::milliseconds>(std::chrono::steady_clock::now() - start_time).count();

bridge/core/executing_context.cc

@@ -877,6 +877,15 @@ void ExecutingContext::MaybeUpdateStyleForFirstPaint() {
   first_paint_committed_ = true;
 }
 
+void ExecutingContext::MaybeCommitFirstPaintStyleSync() {
+  if (!enable_blink_engine_ || !needs_first_paint_style_sync_) {
+    return;
+  }
+
+  needs_first_paint_style_sync_ = false;
+  first_paint_committed_ = true;
+}
+
 void ExecutingContext::FlushUICommand(const BindingObject* self, uint32_t reason) {
   std::vector<NativeBindingObject*> deps;
   FlushUICommand(self, reason, deps);

bridge/core/executing_context.h

@@ -216,6 +216,10 @@ class ExecutingContext {
   FORCE_INLINE bool isIdle() const { return is_idle_; }
   FORCE_INLINE void SetIsIdle(bool is_idle) { is_idle_ = is_idle; }
   FORCE_INLINE void MarkNeedsStyleUpdateInMicrotask() { is_needs_update_styles_in_microtask_ = true; }
+  // If a "first-paint style sync" barrier is active, clear it after we've
+  // performed a synchronous style update so deferred UICommand packages can be
+  // flushed to Dart (e.g. for getComputedStyle()).
+  void MaybeCommitFirstPaintStyleSync();
 
   // Cached media/viewport values pushed from Dart (e.g., during resize). These
   // avoid synchronous GetBindingProperty calls (which may flush layout) during

integration_tests/specs/cssom/first_paint_barrier_sync_flush.ts

@@ -1,5 +1,5 @@
 describe('First-paint barrier: synchronous flush', () => {
-  fit('unblocks getComputedStyle() during RouterLink subtree mount', () => {
+  it('unblocks getComputedStyle() during RouterLink subtree mount', () => {
     const style = document.createElement('style');
     style.textContent = '.sync-flush-box { color: rgb(255, 0, 0); }';
     document.head.appendChild(style);
@@ -26,4 +26,3 @@ describe('First-paint barrier: synchronous flush', () => {
     routerLink.remove();
   });
 });
-