Script using xprop and grep to look for a provided (as cmd-line argument) WM_CLASS. The script identifies a window which has a grandparent with 2 children somewhere in it's tree.

window_finder_bash.sh

#!/bin/bash

# The script is looking for a window with a given WM_CLASS property. The window
# will be attached to the root node (R in the graph below), the root note is
# what you get from 'xprop -root'. There are two different types of windows with
# the same WM_CLASS (Q and W) but their tree structures are different. However,
# the tree structure of the two windows are different as seen in the graph
# below.
#
#     R
#    / \
#   A   Y
#  /\    \
# B  C    X
#     \    \
#      Q    W
#
# To run the script 'xprop' and 'xwininfo' must be installed.
#
# NOTE: This script will not work if a window is not brought to top when it is
# created.

class_name=$1

# regex for extracting hex id's
grep_id='0[xX][a-zA-Z0-9]\{7\}'

##
# Echos the id of the parent to the window with id win_id
# $1 = win_id
function grep_parent_id {
    local result="`xwininfo -tree -id $1 | grep 'Parent window id:' | grep -o $grep_id`"
    echo "$result"
}

# for every change in the root window's _NET_ACTIVE_WINDOW property, which
# is supposed to hold the currently active window (I believe this depends on the WM)
# note that the _NET_ACTIVE_WINDOW event will be triggered if a window that is already
# open is brought to top.
xprop -spy -root _NET_ACTIVE_WINDOW | grep --line-buffered -o $grep_id |
while read -r id; do
    class="`xprop -id $id WM_CLASS | grep $class_name`"
    if [ -n "$class" ]; then
        parent_id=$(grep_parent_id $id)
        grand_parent_id=$(grep_parent_id $parent_id)
        
        grand_parent_tree="`xwininfo -id $grand_parent_id -tree`"
        
        # If the the grand_parent has two children we have found window Q
        tree_check="`grep -o '2 children:' <<< $grand_parent_tree`"
        if [ -z "$tree_check" ]; then
            echo "Found window (W), it has ID: $id"
            # here we could do something awesome ;)
            # if we want to keep track on window W we can spy on it until
            # it's closed.
            xprop -spy -id $id > /dev/null 2>&1
            echo "No more window to listen to"
        else
            echo "Do some thing cool with: $id or start some awseome command"
        fi
    fi
done