While I was learning how the layout of UI elements worked in pygame_gui, I have noticed that whenever I tried to make a window of a specific size, it would always come out smaller than I want it to.

import pygame 
import pygame_gui

pygame.init()

pygame.display.set_caption('Quick Start')
window_surface = pygame.display.set_mode((800, 600))

background = pygame.Surface((800, 600))
background.fill(pygame.Color('#000000'))

manager = pygame_gui.UIManager((800, 600))

ui_window = pygame_gui.elements.UIWindow(rect=pygame.Rect((0, 0), (100, 100)),
                                         window_display_title="UIWindow",
                                         resizable= True)

clock = pygame.time.Clock()
is_running = True

while is_running:
    time_delta = clock.tick(60)/1000.0
    ## print(time_delta)
    print(ui_window.get_abs_rect())

    pygame.draw.rect(background, "blue", pygame.Rect(0, 0, 100, 100)) ## This code draws the rectangle that I would expect the window to occupy
    
    for event in pygame.event.get():
        if event.type == pygame.QUIT:
            is_running = False
            
        manager.process_events(event)

    manager.update(time_delta)

    window_surface.blit(background, (0, 0))
    manager.draw_ui(window_surface)

    pygame.display.update()