[osx] Fix sometimes the floating window flickers during app activation

This solution sends floating window to the background of
the main window on app deactivation to avoid the flickering.

os/osx/app_delegate.mm

@@ -93,8 +93,29 @@ - (void)applicationWillTerminate:(NSNotification*)notification
 
 - (void)applicationWillResignActive:(NSNotification*)notification
 {
-  for (NSWindow* floatingWindow in self.floatingWindows)
-    floatingWindow.level = NSNormalWindowLevel;
+  // Identify the main window
+  NSWindow* mainWindow = nil;
+  for (NSWindow* window in [NSApp windows]) {
+    if (!window.isFloatingPanel) {
+      mainWindow = window;
+      break;
+    }
+  }
+  if (mainWindow) {
+    AppDelegateOSX* delegate = (AppDelegateOSX*)[NSApp delegate];
+    for (NSWindow* floatingWindow in delegate.floatingWindows) {
+      floatingWindow.level = NSNormalWindowLevel;
+      // Send floating window to the background of the main window.
+      // This is done to avoid a small glitch in floating windows when
+      // our application is activated (sometimes the floating window
+      // flickers during application activation). This happens due to
+      // the manipulation of the window 'level' between
+      // 'NSNormalWindowLevel' (inside 'applicationWillResignActive')
+      // and 'NSFloatingWindowLevel' (inside
+      // 'applicationDidBecomeActive').
+      floatingWindow.orderedIndex = mainWindow.orderedIndex + 1;
+    }
+  }
 
   NSEvent* event = [NSApp currentEvent];
   if (event != nil)
@@ -107,7 +128,8 @@ - (void)applicationDidBecomeActive:(NSNotification*)notification
   if (event != nil)
     [ViewOSX updateKeyFlags:event];
 
-  for (NSWindow* floatingWindow in self.floatingWindows)
+  AppDelegateOSX* delegate = (AppDelegateOSX*)[NSApp delegate];
+  for (NSWindow* floatingWindow in delegate.floatingWindows)
     floatingWindow.level = NSFloatingWindowLevel;
 }