mentioned fltk

Do these window managers show PID of running process in window info (xprop):

app         gui   WMs (or names of DEs to be replaced with names of VMs later)
app         gui   wmaker  gnome    kde     lxde    lxqt     xfce
firefox     ?     ?
thunar      gtk
quassel     qt
vlc         ?
libreoffice ?