I just recently moved from gnome2 to Xfce 4. One of the first things I did when I started customizing my set-up, was create a vertical panel on the left side of the screen. This was easy to set-up. However, I noticed that there were a few issues.
The first issue was when I maximized a window, it would maximize part of the window behind the the panel.
The second issue was that when a window was moved over the panel, it would show the panel above the window (screenshot #1 below).
So I started Googling for a solution. I found out that there was quite a lof of other users experiencing the same issues and looking for a fix.
Run the following at the terminal:
xprop -f _NET_WM_WINDOW_TYPE 32a -set _NET_WM_WINDOW_TYPE _NET_WM_WINDOW_TYPE_NORMAL
After running the command above, click on the panel.