As per this ml message:
When an application pops-up a transient, modal window, this should not cause the xfce4-panel to display. This is especially noticeable when the app is in fullscreen because it causes the panel to appear above the app.
To reproduce, open firefox in fullscreen mode and press ctrl-D (bookmark this page). The panel will appear so long as the bookmark window is open.
My svn revisions:
Of course it does, that's on purpose. If an application is fullscreen, it should be sent back on the regular layer if it looses focus, otherwise a blocking window would not be seen.
Ok, that's changed/fixed is SVN
Closing then, minimum revision is 22092, but compositor in svn is still under a big rework and not be stable (yet, hopefully), so you may chosse to wait before upgrading or disable compositor for testing.