Here is how to make the Processing window resizable AND get an event when the window is resized.
Tested with Processing 3.
// For resize event import java.awt.Canvas; import java.awt.event.ComponentEvent; import java.awt.event.ComponentAdapter; void setup() { surface.setResizable(true); ((Canvas)surface.getNative()).addComponentListener(new ComponentAdapter() { @Override public void componentResized(ComponentEvent e) { println("resized " + width + " " + height); } } ); }