How to override paint() method from subclass of frame?

[jingkaimori]

docs/SIP-8.md describes how to override paint method in subclass of Component, but this method cannot apply to subclass of Frame.

[Sciss]

Is it important that it is the frame itself? I think this is a rather low level scenario that was probably not covered for Scala-Swing. You could always install a single component that does the drawing:

import scala.swing._
import java.awt.Color

new Frame {
  contents = new Component {
    preferredSize = new Dimension(400, 400)
    
    override def paintComponent(g: Graphics2D): Unit = {
      g.setColor(Color.red)
      g.drawLine(0, 0, peer.getWidth, peer.getHeight)
    }
  }
  pack().centerOnScreen()
  open()
}

would this be suitable in your case?

[SethTisue]

Note that (not just in Scala Swing, but in Swing generally) every JFrame has a "content pane"; doing things with a JFrame often involves interacting with the content pane, rather than with the frame itself.