fix(ios): run surface transitions on main thread

packages/webgpu/apple/MetalLayerColorSpace.mm

@@ -1,7 +1,6 @@
 #import <CoreGraphics/CoreGraphics.h>
 #import <Foundation/Foundation.h>
 #import <QuartzCore/CAMetalLayer.h>
-#import <QuartzCore/CATransaction.h>
 
 #include "webgpu/webgpu_cpp.h"
 
@@ -31,14 +30,6 @@ void applyCAMetalLayerColorSpace(void *nativeSurface,
     // 8-bit format.
     metalLayer.colorspace = nil;
   }
-  // The change must be set synchronously so the first present already sees
-  // it, and it must reach the render server. On a non-main thread (RN JS or
-  // worklet runtime) the property lands in that thread's implicit
-  // CATransaction, which may never commit on threads without a spinning
-  // runloop, so flush it now. On the main thread the runloop commits it.
-  if (!NSThread.isMainThread) {
-    [CATransaction flush];
-  }
 }
 
 } // namespace rnwgpu

packages/webgpu/cpp/rnwgpu/RNWebGPUManager.cpp

@@ -239,11 +239,19 @@ void RNWebGPUManager::installWebGPUWorkletHelpers(jsi::Runtime &runtime) {
 
 void RNWebGPUManager::flushPendingSurfaceTransition(
     std::shared_ptr<SurfaceInfo> info) {
-  if (info == nullptr || _jsCallInvoker == nullptr) {
+  if (info == nullptr) {
+    return;
+  }
+#ifdef __APPLE__
+  RunOnMainThreadAsync(
+      [info = std::move(info)] { info->applyPendingAttach(); });
+#else
+  if (_jsCallInvoker == nullptr) {
     return;
   }
   _jsCallInvoker->invokeAsync(
       [info = std::move(info)] { info->applyPendingAttach(); });
+#endif
 }
 
 RNWebGPUManager::~RNWebGPUManager() {

packages/webgpu/cpp/rnwgpu/RNWebGPUManager.h

@@ -36,10 +36,9 @@ class RNWebGPUManager {
 
   /**
    * Applies a surface attach latched by the platform UI thread (see
-   * SurfaceInfo::applyPendingAttach) from the JS thread. Surface attaches are
-   * normally adopted at the next frame boundary by whichever thread renders;
-   * this flush covers contexts that are not actively rendering (static
-   * content), so the last offscreen frame still makes it on screen.
+   * SurfaceInfo::applyPendingAttach). Apple runs the transition on main;
+   * other platforms use the JS CallInvoker. This covers contexts that are not
+   * actively rendering, so the last offscreen frame still reaches the screen.
    */
   void flushPendingSurfaceTransition(std::shared_ptr<SurfaceInfo> info);