A small demo of using pywebview with NiceGUI based on wrapping both Unicorn.Server and webview in processes.

nicegui_webview_demo.py

#!/usr/bin/env python3
import multiprocessing
import tempfile

from fastapi import FastAPI
from uvicorn import Config, Server
import webview

from nicegui import ui


class UvicornServer(multiprocessing.Process):

    def __init__(self, config: Config):
        super().__init__()
        self.server = Server(config=config)
        self.config = config

    def stop(self):
        self.terminate()

    def run(self, *args, **kwargs):
        self.server.run()


def start_window(pipe_send, url_to_load):
    def on_closed():
        pipe_send.send('closed')

    win = webview.create_window('Demo', url=url_to_load)
    win.events.closed += on_closed
    webview.start(storage_path=tempfile.mkdtemp())
    
    
app = FastAPI()


@app.get('/')
def read_root():
    return {'Hello': 'World'}


@ui.page('/show')
def show():
    ui.image('https://picsum.photos/id/377/640/360')


ui.run_with(app)


if __name__ == '__main__':
    server_ip = "127.0.0.1"
    server_port = 8080
    conn_recv, conn_send = multiprocessing.Pipe()

    windowsp = multiprocessing.Process(target=start_window, args=(conn_send, f'http://{server_ip}:{server_port}/show'))
    windowsp.start()

    config = Config("main:app", host=server_ip, port=server_port, log_level="debug")
    instance = UvicornServer(config=config)
    instance.start()

    window_status = ''
    while 'closed' not in window_status:
        # get a unit of work
        window_status = conn_recv.recv()
        # report
        print(f'got {window_status}', flush=True)

    instance.stop()

This can obviously be written to be much more functional but the goal was an initial POC to share in the NiceGUI discussions.
They main goal here was co-existance of pywebview and uvicorn (both want to control the execution loop and be the main thread) and making sure the webserver closes when you close the window.

If you want more dynamic control over the webview windows, you'll need to make the pip duplex and implement a control protocol via pipe.