Tkinter grab..

#!/usr/bin/env python3
# -*- coding: utf-8 -*-

import tkinter as tk
from PIL import ImageGrab

class RegionSelector:
    def __init__(self, root):
        self.root = root
        self.root.title("Region Selector")
        self.canvas = tk.Canvas(root, cursor="cross")
        self.canvas.pack(fill=tk.BOTH, expand=True)
        self.select_region_button = tk.Button(root, text="Select Region", command=self.start_selection)
        self.select_region_button.pack(pady=10)
        self.selecting_region = False
        self.start_x = None
        self.start_y = None

    def start_selection(self):
        self.selecting_region = True
        self.root.iconify()
        self.root.update()
        self.canvas.bind("<ButtonPress-1>", self.start_drawing)
        self.canvas.bind("<B1-Motion>", self.draw_marker)
        self.canvas.bind("<ButtonRelease-1>", self.finish_selection)

    def start_drawing(self, event):
        self.start_x = self.canvas.canvasx(event.x)
        self.start_y = self.canvas.canvasy(event.y)

    def draw_marker(self, event):
        if not self.selecting_region:
            return
        current_x = self.canvas.canvasx(event.x)
        current_y = self.canvas.canvasy(event.y)
        self.canvas.delete("marker")
        self.canvas.create_rectangle(self.start_x, self.start_y, current_x, current_y, outline="red", tags="marker")

    def finish_selection(self, event):
        if not self.selecting_region:
            return
        self.selecting_region = False
        x, y, w, h = self.get_coordinates()
        print(f"Selected region: x={x}, y={y}, width={w}, height={h}")
        self.root.deiconify()
        screenshot = ImageGrab.grab(bbox=(x, y, x + w, y + h))
        screenshot.save("capture.png")
        self.canvas.unbind("<ButtonPress-1>")
        self.canvas.unbind("<B1-Motion>")
        self.canvas.unbind("<ButtonRelease-1>")
        self.canvas.delete("marker")

    def get_coordinates(self):
        bbox = self.canvas.bbox("marker")
        if bbox:
            x1, y1, x2, y2 = bbox
            return x1, y1, x2 - x1, y2 - y1
        else:
            return 0, 0, 0, 0

if __name__ == "__main__":
    root = tk.Tk()
    root.geometry("400x300")
    app = RegionSelector(root)
    root.mainloop()