Show macOS transient and standalone windows nicely centered

dgl/src/WindowPrivateData.cpp

@@ -316,8 +316,10 @@ void Window::PrivateData::show()
         PuglRect rect = puglGetFrame(view);
         puglSetWindowSize(view, static_cast<uint>(rect.width), static_cast<uint>(rect.height));
 
-#ifdef DISTRHO_OS_WINDOWS
-        puglWin32ShowWindowCentered(view);
+#if defined(DISTRHO_OS_WINDOWS)
+        puglWin32ShowCentered(view);
+#elif defined(DISTRHO_OS_MAC)
+        puglMacOSShowCentered(view);
 #else
         puglShow(view);
 #endif

dgl/src/pugl.cpp

@@ -501,6 +501,33 @@ puglMacOSRemoveChildWindow(PuglView* const view, PuglView* const child)
     return PUGL_FAILURE;
 }
 
+// --------------------------------------------------------------------------------------------------------------------
+// macOS specific, center view based on parent coordinates (if there is one)
+
+void puglMacOSShowCentered(PuglView* const view)
+{
+    if (puglShow(view) != PUGL_SUCCESS)
+        return;
+
+    if (view->transientParent != 0)
+    {
+        NSWindow* const transientWindow = [(NSView*)view->transientParent window];
+        DISTRHO_SAFE_ASSERT_RETURN(transientWindow != nullptr,);
+
+        const NSRect ourFrame       = [view->impl->window frame];
+        const NSRect transientFrame = [transientWindow frame];
+
+        const int x = transientFrame.origin.x + transientFrame.size.width / 2 - ourFrame.size.width / 2;
+        const int y = transientFrame.origin.y + transientFrame.size.height / 2  + ourFrame.size.height / 2;
+
+        [view->impl->window setFrameTopLeftPoint:NSMakePoint(x, y)];
+    }
+    else
+    {
+        [view->impl->window center];
+    }
+}
+
 // --------------------------------------------------------------------------------------------------------------------
 // macOS specific, setup file browser dialog
 
@@ -564,7 +591,7 @@ void puglWin32RestoreWindow(PuglView* const view)
 // --------------------------------------------------------------------------------------------------------------------
 // win32 specific, center view based on parent coordinates (if there is one)
 
-void puglWin32ShowWindowCentered(PuglView* const view)
+void puglWin32ShowCentered(PuglView* const view)
 {
     PuglInternals* impl = view->impl;
     DISTRHO_SAFE_ASSERT_RETURN(impl->hwnd != nullptr,);

dgl/src/pugl.hpp

@@ -108,6 +108,10 @@ puglMacOSAddChildWindow(PuglView* view, PuglView* child);
 PUGL_API PuglStatus
 puglMacOSRemoveChildWindow(PuglView* view, PuglView* child);
 
+// macOS specific, center view based on parent coordinates (if there is one)
+PUGL_API void
+puglMacOSShowCentered(PuglView* view);
+
 // macOS specific, setup file browser dialog
 typedef void (*openPanelCallback)(PuglView* view, const char* path);
 bool puglMacOSFilePanelOpen(PuglView* view, const char* startDir, const char* title, uint flags, openPanelCallback callback);
@@ -120,7 +124,7 @@ puglWin32RestoreWindow(PuglView* view);
 
 // win32 specific, center view based on parent coordinates (if there is one)
 PUGL_API void
-puglWin32ShowWindowCentered(PuglView* view);
+puglWin32ShowCentered(PuglView* view);
 
 // win32 specific, set or unset WS_SIZEBOX style flag
 PUGL_API void