A small Python script that creates a semi-transparent resizable frame that stays on top. I use it to mark an area of my screen (the area where my webcam is overlayed) off-limits when screencasting.

frame.py

#!/usr/bin/env python

# shamelessly stolen and adapted from https://gist.github.com/KurtJacobson/374c8cb83aee4851d39981b9c7e2c22c

# usage: python3 frame.py [width] [height] [x] [y] [opacity]

import cairo
import gi
import sys
import resource

gi.require_version('Gtk', '3.0')
gi.require_version('Gdk', '3.0')

from gi.repository import Gtk
from gi.repository import Gdk

class TransparentWindow(Gtk.Window):
    def __init__(self):
        Gtk.Window.__init__(self)

        if len(sys.argv) >= 3:
            w = int(sys.argv[1])
            h = int(sys.argv[2])
        else:
            w = 400
            h = 300
        self.set_size_request(w, h)

        if len(sys.argv) >= 5:
            x = int(sys.argv[3])
            y = int(sys.argv[4])
            self.move(x, y)

        self.set_title('')
        self.set_skip_taskbar_hint(True)

        self.connect('destroy', Gtk.main_quit)
        self.connect('draw', self.draw)

        screen = self.get_screen()
        visual = screen.get_rgba_visual()
        if visual and screen.is_composited():
            self.set_visual(visual)

        self.set_app_paintable(True)
        self.show_all()

        self.set_keep_above(True)

    def draw(self, widget, context):
        if len(sys.argv) >= 6:
            opacity = float(sys.argv[5])
        else:
            opacity = 0.1
        context.set_source_rgba(1, 1, 1, opacity)
        context.set_operator(cairo.OPERATOR_SOURCE)
        context.paint()
        context.set_operator(cairo.OPERATOR_OVER)
        #print(self.get_size())

TransparentWindow()
Gtk.main()