diff --git a/src/detail/auv3/auv3_audiounit.h b/src/detail/auv3/auv3_audiounit.h
index 98212c4b..9a790d24 100644
--- a/src/detail/auv3/auv3_audiounit.h
+++ b/src/detail/auv3/auv3_audiounit.h
@@ -33,6 +33,10 @@
 // Subclasses (generated by build-helper) override this to provide plugin-specific info
 - (AUAudioUnit *)createAudioUnitWithComponentDescription:(AudioComponentDescription)desc
                                                    error:(NSError **)error;
+// The generated override must build its audio unit through this: the host's
+// factory call and -loadView's self-bootstrap race, and only one audio unit may
+// ever exist per view controller. `make` runs only if there is none yet.
+- (AUAudioUnit *)createAudioUnitOnce:(ClapAUv3AudioUnit * (^)(NSError **))make error:(NSError **)error;
 @end
 
 @interface ClapAUv3AudioUnit : AUAudioUnit
diff --git a/src/detail/auv3/auv3_audiounit.mm b/src/detail/auv3/auv3_audiounit.mm
index cd7c6183..715aee23 100644
--- a/src/detail/auv3/auv3_audiounit.mm
+++ b/src/detail/auv3/auv3_audiounit.mm
@@ -2077,7 +2077,13 @@ - (BOOL)setGUISize:(uint32_t)width height:(uint32_t)height
   // misbehaviour.
   auto *gui = _impl->_plugin->_ext._gui;
   if (gui->adjust_size) gui->adjust_size(_impl->_plugin->_plugin, &width, &height);
-  return gui->set_size(_impl->_plugin->_plugin, width, height) ? YES : NO;
+  if (!gui->set_size(_impl->_plugin->_plugin, width, height)) return NO;
+
+  // Keep the cached size in step: prepareGUIAndReturnWidth:height: hands it
+  // back, and that is how the view controller learns the adjusted size.
+  _impl->_guiWidth = width;
+  _impl->_guiHeight = height;
+  return YES;
 }
 
 - (void)setViewController:(ClapAUv3ViewController *)vc
@@ -2277,6 +2283,8 @@ @implementation ClapAUv3ViewController
   CGSize _pluginSize;
   // We already tried to create the audio unit ourselves (see _bootstrapAudioUnit).
   BOOL _bootstrapAttempted;
+  // Serializes the two creation paths (see -createAudioUnitOnce:error:).
+  std::mutex _audioUnitMutex;
 }
 
 - (void)loadView
@@ -2309,6 +2317,10 @@ - (void)loadView
 
 - (void)setAudioUnit:(ClapAUv3AudioUnit *)audioUnit
 {
+  if (_audioUnit == audioUnit) return;
+  // Never drop a live audio unit by bare overwrite: releasing the last reference
+  // reaches plugin->terminate() with the GUI still created, which CLAP forbids.
+  if (_audioUnit) [self _destroyGUI];
   _audioUnit = audioUnit;
   // Establish the back-reference so the AU can return us from
   // requestViewControllerWithCompletionHandler:
@@ -2362,6 +2374,22 @@ - (BOOL)_ownComponentDescription:(AudioComponentDescription *)outDesc
   return NO;
 }
 
+// Both creation paths meet here: the host's factory call, which arrives on an
+// XPC worker thread, and -loadView's self-bootstrap on the appex main thread.
+// Unserialized they both see a nil audioUnit and build an audio unit, and the
+// loser is then thrown away with its GUI still alive. Nothing under this lock
+// ever waits on the main queue, so the losing path only stalls.
+- (AUAudioUnit *)createAudioUnitOnce:(ClapAUv3AudioUnit * (^)(NSError **))make error:(NSError **)error
+{
+  std::lock_guard<std::mutex> lock(_audioUnitMutex);
+  if (_audioUnit) return _audioUnit;
+
+  ClapAUv3AudioUnit *au = make(error);
+  if (!au) return nil;
+  self.audioUnit = au;
+  return au;
+}
+
 // Out-of-process AUv3: the viewbridge loads our view from inside
 // -beginRequestWithExtensionContext:, before the host has asked the factory for
 // an audio unit, and it captures preferredContentSize as the view is vended —
@@ -2565,10 +2593,20 @@ - (void)viewDidLayoutSubviews
       [self.audioUnit setGUISize:(uint32_t)bounds.size.width height:(uint32_t)bounds.size.height];
     }
 
-    // Ensure the CLAP plugin's subview fills the container
+    // adjust_size may have snapped the container bounds (fixed aspect ratio,
+    // step sizes), and the plugin laid out at the snapped size — so the subview
+    // has to get that size too, not the raw bounds.
+    CGRect childFrame = bounds;
+    uint32_t agreedWidth = 0, agreedHeight = 0;
+    if ([self.audioUnit prepareGUIAndReturnWidth:&agreedWidth height:&agreedHeight] && agreedWidth > 0 &&
+        agreedHeight > 0)
+    {
+      childFrame = CGRectMake(0, 0, agreedWidth, agreedHeight);
+    }
+
     for (CLAPWRAP_ViewClass *subview in self.view.subviews)
     {
-      subview.frame = bounds;
+      subview.frame = childFrame;
     }
   }
 }
diff --git a/src/detail/auv3/build-helper/build-helper.cpp b/src/detail/auv3/build-helper/build-helper.cpp
index 8b878a6f..7997cfd9 100644
--- a/src/detail/auv3/build-helper/build-helper.cpp
+++ b/src/detail/auv3/build-helper/build-helper.cpp
@@ -481,19 +481,18 @@ int main(int argc, char **argv)
       cppf
           << "- (AUAudioUnit *)createAudioUnitWithComponentDescription:(AudioComponentDescription)desc\n"
           << "                                                   error:(NSError **)error {\n"
-          << "    // The view controller may have created it already, to answer a size\n"
-          << "    // query before the host got here (see -_bootstrapAudioUnit).\n"
-          << "    if (self.audioUnit) return self.audioUnit;\n"
-          << "    ClapAUv3AudioUnit *au = [[ClapAUv3AudioUnit alloc] initWithComponentDescription:desc\n"
+          << "    // -loadView may be bootstrapping one right now on the main thread, so the\n"
+          << "    // check and the build have to happen under the base class's lock.\n"
+          << "    return [self createAudioUnitOnce:^ClapAUv3AudioUnit *(NSError **err) {\n"
+          << "        return [[ClapAUv3AudioUnit alloc] initWithComponentDescription:desc\n"
           << "                                                          options:0\n"
-          << "                                                            error:error\n"
+          << "                                                            error:err\n"
           << "                                                         clapName:@\""
           << escapeObjCString(u.clapname) << "\"\n"
           << "                                                           clapId:@\""
           << escapeObjCString(u.clapid) << "\"\n"
           << "                                                        clapIndex:" << idx << "];\n"
-          << "    self.audioUnit = au;\n"
-          << "    return au;\n"
+          << "    } error:error];\n"
           << "}\n"
           << "@end\n\n";
 
diff --git a/src/detail/auv3/templates/generated_auv3_entrypoints.hxx.in b/src/detail/auv3/templates/generated_auv3_entrypoints.hxx.in
index 5592236d..2c730533 100644
--- a/src/detail/auv3/templates/generated_auv3_entrypoints.hxx.in
+++ b/src/detail/auv3/templates/generated_auv3_entrypoints.hxx.in
@@ -13,16 +13,15 @@
 @implementation @AUV3_FACTORY_CLASS_NAME@
 - (AUAudioUnit *)createAudioUnitWithComponentDescription:(AudioComponentDescription)desc
                                                    error:(NSError **)error {
-    // The view controller may have created it already, to answer a size
-    // query before the host got here (see -_bootstrapAudioUnit).
-    if (self.audioUnit) return self.audioUnit;
-    ClapAUv3AudioUnit *au = [[ClapAUv3AudioUnit alloc] initWithComponentDescription:desc
+    // -loadView may be bootstrapping one right now on the main thread, so the
+    // check and the build have to happen under the base class's lock.
+    return [self createAudioUnitOnce:^ClapAUv3AudioUnit *(NSError **err) {
+        return [[ClapAUv3AudioUnit alloc] initWithComponentDescription:desc
                                                           options:0
-                                                            error:error
+                                                            error:err
                                                          clapName:@"@AUV3_CLAP_NAME@"
                                                            clapId:@"@AUV3_CLAP_ID@"
                                                         clapIndex:0];
-    self.audioUnit = au;
-    return au;
+    } error:error];
 }
 @end