I open a NSWindow from my main NSWindow.
DropHereWindowController *dropHereWindowController = [[DropHereWindowController alloc] initWithWindowNibName:@\"Drop
NSStatusWindowLevel works, just use it after some small delay after starting the app (or after creating the window).
NSStatusWindowLevel
dispatch_after(dispatch_time(DISPATCH_TIME_NOW, (int64_t)(1 * NSEC_PER_SEC)), dispatch_get_main_queue(), ^ { self.view.window.level = NSStatusWindowLevel; });