Why NSWindow without styleMask:NSTitledWindowMask can not be keyWindow?

Question

Problem: I have one window mainWindow and another childWindow added to mainWindow. childWindow is kind of WindowExt class. This class

Answer 1

That’s a Cocoa design decision: windows without title or resize bar cannot become key window by default.

If you want a titleless window to be able to become a key window, you need to create a subclass of NSWindow and override -canBecomeKeyWindow as follows:

- (BOOL)canBecomeKeyWindow {
    return YES;
}

Answer 2

you can set the style mask to NSBorderlessWindowMask in order to make it borderless