Did I tell you so?
I've noted many times that an X client that tries to reparent the window of another X client without the assistance of the other client or the window manager is inherently flawed. X doesn't work that way.
This is why the KDE system tray protocol often failed outside of KDE. It depended on the window manager, but just how was not known to many. (With WM support, it worked fine. It's another question whether it should depend on the window manager.) GNOME's status dock (the old one) was an acknowledged hack that failed more frequently as window managers improved but without support for KDE's protocol.
The "swallowing" feature of the GNOME panel exhibited the same error. Perhaps because this was finally realized, the feature was removed. It has been resurrected as an applet, but it is still doomed.
Now, prompting this entry, Mozplugger has failed as I expected. Although it certainly tries harder than the old GNOME status dock or any "swallowing" for which I've read the code, it too is ultimately doomed. Moments ago, a GhostView window created from a process launched by Mozplugger appeared in the bottom right corner of my screen and stayed there.
I wonder who will get the first bug report. It belongs to Mozplugger, but I won't be filing it. There's no point to filing a bug against a program for the basic concept of the program, is there? I imagine there will be reports to every program possibly involved: browsers; the browser engine; the to-be-reparented programs; window managers; the GNOME panel; the GNOME window list; libwnck (these last three because a button for the window appears in the window list in the panel); and, of course, the culprit, Mozplugger. Despite this entry, fingers will be pointed and tongues will be forked.
Providence, I should say nothing more.