fix(bridge): VSTi GUI crash on instrument switch — PostMessage WM_CLOSE instead of cross-thread DestroyWindow

- LOAD job posts WM_CLOSE to window created on main thread (DestroyWindow from worker crashed 0xc000041d, muting all tracks)
- handleOpenGui + attach jobs skip when Vst3Instrument::hasAttachedView() (state_ && guiAttached_)
- attach failure posts WM_CLOSE; post_close_gui erases entry only when it points at the same hwnd
- cap openVstGuiRetry at 5 to stop frontend retry spam

native_bridge/src/Vst3Instrument.cpp

@@ -635,6 +635,10 @@ bool Vst3Instrument::attachView(void* parentWindowHandle) {
 #endif
 }
 
+bool Vst3Instrument::hasAttachedView() const {
+    return state_ != nullptr && guiAttached_;
+}
+
 bool Vst3Instrument::reload() {
 #ifndef HAVE_VST3SDK
     return false;

native_bridge/src/main.cpp

@@ -197,12 +197,17 @@ static void post_close_gui(uint32_t ch, HWND hwnd) {
             if (auto* i = g_engine->get(ch)) i->closeGUI();
             std::lock_guard<std::mutex> lock(g_guiMutex);
             g_hwndToCh.erase(hwnd);
-            g_guiWindows.erase(ch);
+            // Chi erase neu map van tro den HWND NAY — neu reopen da tao
+            // window moi (race: close cu + OPEN_GUI moi), khong duoc xoa
+            // entry cua window moi (con tro treo -> leak window moi).
+            auto wit = g_guiWindows.find(ch);
+            if (wit != g_guiWindows.end() && wit->second == hwnd) g_guiWindows.erase(wit);
         });
     } else {
         std::lock_guard<std::mutex> lock(g_guiMutex);
         g_hwndToCh.erase(hwnd);
-        g_guiWindows.erase(ch);
+        auto wit = g_guiWindows.find(ch);
+        if (wit != g_guiWindows.end() && wit->second == hwnd) g_guiWindows.erase(wit);
     }
 }
 
@@ -361,6 +366,16 @@ int main(int argc, char* argv[]) {
             g_pendingGui[guiCh] = { (void*)arg1, pluginId };
             return;
         }
+        // Dedupe OPEN_GUI spam (frontend openVstGuiRetry): view dang
+        // attached -> GUI da mo, khong post attach job lan nua. Re-attach
+        // tren view dang attached lam plugin loi (Nexus createView null).
+        if (auto* i0 = instruments.get(guiCh)) {
+            if (i0->hasAttachedView()) {
+                std::cerr << "[dbg] openGUI: view already attached ch=" << guiCh
+                          << " — skip" << std::endl;
+                return;
+            }
+        }
         if (!workers[guiCh]) workers[guiCh] = std::make_unique<ChannelWorker>();
         void* hwnd = (void*)arg1;
 #ifdef _WIN32
@@ -405,6 +420,13 @@ int main(int argc, char* argv[]) {
             }
             std::cerr << "[dbg] openGUI thread start hwnd=" << hwnd
                       << " plugin=" << arg2 << " ch=" << guiCh << std::endl;
+            if (auto* inst0 = instruments.get(guiCh)) {
+                if (inst0->hasAttachedView()) {
+                    std::cerr << "[dbg] openGUI: view already attached ch=" << guiCh
+                              << " — skip" << std::endl;
+                    return;
+                }
+            }
 #ifdef _WIN32
             // Option B: chi 1 editor VST mo tai 1 thoi diem toan
             // bridge. Instance thu 2 cua CUNG plugin (Nexus) attach
@@ -468,9 +490,17 @@ int main(int argc, char* argv[]) {
                 if (ok)
                     std::cout << "[NativeBridge] GUI attached hwnd=" << hwnd
                               << " plugin=" << arg2 << " ch=" << guiCh << std::endl;
-                else
+                else {
                     std::cerr << "[NativeBridge] GUI attach FAILED hwnd=" << hwnd
                               << " plugin=" << arg2 << std::endl;
+#ifdef _WIN32
+                    // Attach that — khong co view (VD: channel la SF2/SFZ hoac plugin
+                    // loi). Dong ngay cua so vo nghia de khong con window treo trong
+                    // registry; WM_CLOSE -> main pump destroy tren main thread (owner).
+                    // PostMessage an toan cross-thread (khong nhu DestroyWindow).
+                    PostMessageA((HWND)hwnd, WM_CLOSE, 0, 0);
+#endif
+                }
             }
         });
     };
@@ -513,10 +543,13 @@ int main(int argc, char* argv[]) {
                     // DONG cua so editor dang mo cua channel TRUOC khi assign():
                     // thay the inst (VST3 -> SF2/inst khac) ma editor con song ->
                     // old inst destructor goi view->removed() tren HWND con hoat
-                    // dong -> plugin block -> treo bridge. DestroyWindow chay tren
-                    // CUNG worker thread so huu window; WM_DESTROY goi closeGUI()
-                    // dung thu tu. Phai erase map TRUOC DestroyWindow (WM_DESTROY
-                    // handler lay g_guiMutex lai — khong duoc giu lock khi destroy).
+                    // dong -> plugin block -> treo bridge.
+                    // QUAN TRONG: window duoc tao tren MAIN thread (handleOpenGui
+                    // goi create_native_vst_window o main loop) — DestroyWindow tu
+                    // worker thread la cross-thread (MSDN cam), gay crash USER32
+                    // 0xc000041d. Post WM_CLOSE -> main pump destroy window tren
+                    // DUNG thread so huu no. Registry da erase o tren nen WM_DESTROY
+                    // khong goi closeGUI tren inst cu (inst moi chua co view).
                     HWND hToDestroy = nullptr;
                     {
                         std::lock_guard<std::mutex> lock(g_guiMutex);
@@ -527,7 +560,7 @@ int main(int argc, char* argv[]) {
                             g_hwndToCh.erase(hToDestroy);
                         }
                     }
-                    if (hToDestroy && IsWindow(hToDestroy)) DestroyWindow(hToDestroy);
+                    if (hToDestroy && IsWindow(hToDestroy)) PostMessageA(hToDestroy, WM_CLOSE, 0, 0);
 #endif
                     std::cerr << "[dbg] load thread start ch=" << ch
                               << " type=" << (int)t << " path=" << path << std::endl;