Artifacts after corner rounding, pywin32

Question

I have minimal code in python kivy + pywin32. I use the CreateRoundRectRgn function to create a region with rounded corners, and then assign this region to my window via SetWindowRgn. How to remove artifacts on corners?

Here are the artifacts

import os
import win32con
import win32gui
import win32api

os.environ['PYTHONFAULTHANDLER'] = '1'

from additive import createDirs, writer

createDirs(["config",], True)
writer(0, 1)

from kivy.app import App
from kivy.core.window import Window
from kivy.uix.screenmanager import ScreenManager, FadeTransition
from kivy.lang.builder import Builder
from kivy.uix.floatlayout import FloatLayout

def resource_path(relative_path):
    try:
        base_path = sys._MEIPASS
    except Exception:
        base_path = os.path.abspath(".")

    return os.path.join(base_path, relative_path)

TemplateDir = resource_path('styles/')
for file in os.listdir(TemplateDir):
    Builder.load_file(TemplateDir+file)

from LocalScreen import LocalScreen as LS

from userWidgets import TB

class FastMessageApp(App):
    def __init__(self, **kwargs):
        super(FastMessageApp, self).__init__(**kwargs)

        Window.clearcolor = (0.1, 0.1, 0.1, 1)
        Window.size = (500, 400)
        Window.set_title("FastMessage")

        self.HWND = win32gui.FindWindow(None, "FastMessage")
        self.RoundedRegion = win32gui.CreateRoundRectRgn(0, 0, 500, 400, 50, 50)
        win32gui.SetWindowRgn(self.HWND, self.RoundedRegion, True)
        win32gui.SetWindowLong(self.HWND, win32con.GWL_EXSTYLE, win32gui.GetWindowLong(self.HWND, win32con.GWL_EXSTYLE) | win32con.WS_EX_LAYERED)
        win32gui.SetLayeredWindowAttributes(self.HWND, win32api.RGB(0,0,0), 255, win32con.LWA_ALPHA)

        self.__Manager = ScreenManager(transition = FadeTransition(duration=0.1), size_hint = (1,0.95), pos_hint = {'bottom': 1})

    def build(self):
        self.__Window = FloatLayout()
        self.__Window.add_widget(TB())
        self.__Window.add_widget(self.__Manager)
        return self.__Window

if __name__ == "__main__":
    app = FastMessageApp()
    app.run()

I tried to reduce the radius of the rounding, but the size of the artifacts remained the same, although there were fewer of them.

here

I also tried to call the ExtSelectClipRgn function from GDI via ctypes, but none of the flags helped me.