[Python] Why is nothing drawn in Pygame at all?

I have started a new project in python using Pygame and for the background I want the bottom half filled with gray and the top black. I have used rect drawing in projects before, but for some reason it seems to be broken? I don't know what I am doing wrong.

The weirdest thing is that the result is different every time I run the program. Sometimes there is only a black screen and sometimes a gray rectangle covers part of the screen, but never half of the screen.

import pygame, sys
from pygame.locals import *
pygame.init()

DISPLAY = pygame.display.set_mode((800, 800))
pygame.display.set_caption("thing")
pygame.draw.rect(DISPLAY, (200, 200, 200), pygame.Rect(0, 400, 800, 400))

while True:
for event in pygame.event.get():
if event.type == QUIT:
pygame.quit()
sys.exit()

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