I open a NSWindow from my main NSWindow.
DropHereWindowController *dropHereWindowController = [[DropHereWindowController alloc] initWithWindowNibName:@\"Drop
Using the CGWindowLevelKey kCGMaximumWindowLevelKey also works.
CGWindowLevelKey
kCGMaximumWindowLevelKey
[window setLevel:CGWindowLevelForKey(kCGMaximumWindowLevelKey)];
CGWindowLevelKey Reference