pip install flaskwebgui

from flask import Flask  
from flaskwebgui import FlaskUI #get the FlaskUI class


app = Flask(__name__)

# Feed it the flask app instance (check bellow what param you can add)
ui = FlaskUI(app) 


# do your logic as usual in Flask

@app.route("/")
def index():  
    return "It works!"



# call the 'run' method
ui.run()
 

Default FlaskUI class parameters:

• app, ==> flask class instance (required)
• width=800 ==> default width 800
• height=600 ==> default height 600
• browser_path="", ==> full path to browser.exe ("C:/browser_folder/chrome.exe") (needed if you want to start a specific browser)
• server="flask" ==> the default backend framework is flask, but you can add a function which starts the desired server for your choosed framework (bottle, django, web2py pyramid etc)
• host="127.0.0.1" ==> specify other if needed
• port=5000 ==> specify other if needed

Should work on windows/linux/mac with no isssues.

If browser is not found, this would be quick fix: FlaskUI (app, browser_path=r"path/to/chrome.exe")

• Download portable Chromium for the your targeted os

• place the extracted portable app next to "main.py" file,

• flaskwebgui will look for chrome.exe/.app/.sh

In this way when you distribute it, you don't need users to have chrome installed, it will work like a portable app.
Also, during development of your app do that in the normal way you do a Flask app without flaskwebgui. Use flaskwebgui only when you are finished the app(test the app) and ready to deploy.
flaskwebgui doesn't interfere with your way of doing a flask application it just helps converting it into a desktop app more easily with pyinstaller or pyvan

You can distribute it as a standalone desktop app with pyinstaller or pyvan.

It's a combination of https://github.com/Widdershin/flask-desktop and https://github.com/ChrisKnott/Eel
flaskwebgui just uses threading to start a flask server and the browser in app mode (for chrome)
It has some advantages over flask-desktop because it doesn't use PyQt5, so you won't have any issues regarding licensing and over Eel because you don't need to learn any logic other than Flask.