fix styling and add spec

atom/browser/api/atom_api_browser_window.cc

@@ -1086,7 +1086,7 @@ void BrowserWindow::AddTabbedWindow(NativeWindow* window,
                                     mate::Arguments* args) {
   const bool windowAdded = window_->AddTabbedWindow(window);
   if (!windowAdded)
-    args->ThrowError("AddTabbedWindow cannot be called by a window on itself");
+    args->ThrowError("AddTabbedWindow cannot be called by a window on itself.");
 }
 
 void BrowserWindow::SetVibrancy(mate::Arguments* args) {

atom/browser/native_window_mac.mm

@@ -918,7 +918,6 @@ NativeWindowMac::NativeWindowMac(
   }
 
   if (transparent()) {
-    NSLog(@"Setting transparent");
     // Setting the background color to clear will also hide the shadow.
     [window_ setBackgroundColor:[NSColor clearColor]];
   }

spec/api-browser-window-spec.js

@@ -733,6 +733,12 @@ describe('BrowserWindow module', () => {
         done()
       })
     })
+
+    it('throws when called on itself', () => {
+      assert.throws(() => {
+        w.addTabbedWindow(w)
+      }, /AddTabbedWindow cannot be called by a window on itself./)
+    })
   })
 
   describe('BrowserWindow.setVibrancy(type)', () => {