```Swift
import Cocoa

class OverlayWindow: NSWindow {
    var targetWindow: NSWindow?

    init() {
        super.init(contentRect: NSZeroRect, styleMask: [.borderless], backing: .buffered, defer: false)
        backgroundColor = NSColor.clear
        isOpaque = false
        level = .floating
    }

    func attach(to window: NSWindow) {
        targetWindow = window
        targetWindow?.addChildWindow(self, ordered: .above)
        setFrame((targetWindow?.frame)!, display: true)
    }

    override func mouseDown(with event: NSEvent) {
        // Handle mouse events as needed
    }

    override func keyDown(with event: NSEvent) {
        // Handle keyboard events as needed
    }

    override func draw(_ dirtyRect: NSRect) {
        // Draw your overlay content here using Core Graphics
        let path = NSBezierPath(rect: bounds.insetBy(dx: 10, dy: 10))
        NSColor.blue.set()
        path.fill()
    }
}
```

The code above shows how to create an overlay window in Swift for macOS using the Core Graphics framework. The `OverlayWindow` class inherits from `NSWindow` and provides a method `attach(to:)` that allows you to attach the overlay window to the target window. You can then handle mouse and keyboard events as needed.

Overall, this is a useful starting point for creating an overlay window in Swift. You may need to add additional features depending on your specific use case, but this code provides a solid foundation to build upon.
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