A simple example demonstrating how to move GTK+ widgets with the mouse.

move_widget.py

#!/usr/bin/env python

import gi

gi.require_version('Gtk', '3.0')
from gi.repository import Gtk

class MoveButton(Gtk.Fixed):

    def __init__(self):
        Gtk.Fixed.__init__(self)

        # Initial event position
        self.initial_x = 0
        self.initial_y = 0

        # Initial button position
        self.initial_pos_x = 0
        self.initial_pos_y = 0


    # Called on "drag start"
    def button_press_event(self, widget, event):

        # Get the parent window size
        pw = self.get_allocation().width
        ph = self.get_allocation().height

        # Get the button location and size
        x = self.child_get_property(widget, 'x')
        y = self.child_get_property(widget, 'y')
        w = widget.get_allocation().width
        h = widget.get_allocation().height

        # Calculate the maximum change in position such that the
        # button does not move outside the bounds of its parent
        self.dx_max = pw - (x + w)
        self.dy_max = ph - (y + h)

        # Save the initial values
        self.initial_x = event.x_root
        self.initial_y = event.y_root
        self.initial_pos_x = x
        self.initial_pos_y = y

    # Called on "drag motion"
    def motion_notify_event(self, widget, event):

        # Calculate the change in position of the pointer
        dx = event.x_root - self.initial_x
        dy = event.y_root - self.initial_y

        # Determine the new position such that the button
        # does not move outside the bounds of its parent
        x = self.initial_pos_x + max(min(dx, self.dx_max), -self.initial_pos_x)
        y = self.initial_pos_y + max(min(dy, self.dy_max), -self.initial_pos_y)

        # Actualy move the button
        self.child_set_property(widget, 'x', x)
        self.child_set_property(widget, 'y', y)


def make_button(text):

    # Helper function to make the buttons
    b = Gtk.Button.new_with_label(text)
    b.connect("button_press_event", move_btn.button_press_event)
    b.connect("motion_notify_event", move_btn.motion_notify_event)
    b.show()
    return b

# New top level window
window = Gtk.Window()
window.connect("destroy", Gtk.main_quit)

# Initialize the MoveButton container
move_btn = MoveButton()
window.add(move_btn)

# Add some buttons
move_btn.put(make_button("A button"), 50, 50)
move_btn.put(make_button("Another button"), 250, 100)

window.show_all()

# Start the main loop
Gtk.main()