main.cpp

#include "ofMain.h"
#include "ofApp.h"

//========================================================================
int main( ){
	ofGLFWWindowSettings windowSettings;
	windowSettings.glVersionMajor = 4;
	windowSettings.glVersionMinor = 0;

	auto window = ofCreateWindow(windowSettings);
	ofRunApp(new ofApp());

}

ofApp.cpp

#include "ofApp.h"

//--------------------------------------------------------------
void ofApp::setup(){
	//setup an fbo
	{
		ofFbo::Settings fboSettings;
		fboSettings.width = 512;
		fboSettings.height = 512;
		fboSettings.internalformat = GL_RGBA32F;
		this->fbo.allocate(fboSettings);
	}

	//setup the window
	{
		ofGLFWWindowSettings windowSettings;
		windowSettings.shareContextWith = ofGetCurrentWindow();
		windowSettings.width = 640;
		windowSettings.height = 480;
		windowSettings.setPosition({ 1000, 40 });

		this->window = make_shared<ofAppGLFWWindow>();
		this->window->setup(windowSettings);
		this->window->update();
	}
}

//--------------------------------------------------------------
void ofApp::update(){
	//draw to the fbo
	{
		this->fbo.begin();
		{
			ofClear(0, 255);

			ofPushStyle();
			{
				ofSetColor(255, 100, 100);
				ofDrawCircle(30, 30, 100);
			}
			ofPopStyle();

			ofDrawBitmapStringHighlight("this is the fbo", 10, 10);
		}
		this->fbo.end();
	}

	//cache the app window for later
	auto appWindow = ofGetCurrentWindow();

	//draw the window
	window->update();
	window->makeCurrent();
	{
		window->renderer()->startRender();
		ofClear(0, 0);
		ofPushView();
		{
			ofSetupScreenOrtho();
			ofDrawCircle(50, 50, 50);
			ofDrawBitmapStringHighlight("this is the client window", 20, 20);
			this->fbo.draw(20, 50);
		}
		ofPopView();
		window->swapBuffers();
		window->renderer()->finishRender();
	}
	appWindow->makeCurrent();

	ofDrawBitmapStringHighlight("this is the main window in update", 20, 20);
}

//--------------------------------------------------------------
void ofApp::draw(){
	ofDrawBitmapStringHighlight("this is the new main window in draw", 20, 30);

	fbo.draw(20, 50);
}

//--------------------------------------------------------------
void ofApp::keyPressed(int key){

}

//--------------------------------------------------------------
void ofApp::keyReleased(int key){

}

//--------------------------------------------------------------
void ofApp::mouseMoved(int x, int y ){

}

//--------------------------------------------------------------
void ofApp::mouseDragged(int x, int y, int button){

}

//--------------------------------------------------------------
void ofApp::mousePressed(int x, int y, int button){

}

//--------------------------------------------------------------
void ofApp::mouseReleased(int x, int y, int button){

}

//--------------------------------------------------------------
void ofApp::mouseEntered(int x, int y){

}

//--------------------------------------------------------------
void ofApp::mouseExited(int x, int y){

}

//--------------------------------------------------------------
void ofApp::windowResized(int w, int h){

}

//--------------------------------------------------------------
void ofApp::gotMessage(ofMessage msg){

}

//--------------------------------------------------------------
void ofApp::dragEvent(ofDragInfo dragInfo){ 

}

ofApp.h

#pragma once

#include "ofMain.h"

class ofApp : public ofBaseApp{

	public:
		void setup();
		void update();
		void draw();

		void keyPressed(int key);
		void keyReleased(int key);
		void mouseMoved(int x, int y );
		void mouseDragged(int x, int y, int button);
		void mousePressed(int x, int y, int button);
		void mouseReleased(int x, int y, int button);
		void mouseEntered(int x, int y);
		void mouseExited(int x, int y);
		void windowResized(int w, int h);
		void dragEvent(ofDragInfo dragInfo);
		void gotMessage(ofMessage msg);
		
		shared_ptr<ofAppBaseWindow> window;
		ofFbo fbo;
		ofTexture textureCopy;
};