Created attachment 2155 Screenshot. Lower left corner (red/white arrow) shows the last visible icon. If there are many icons in the iconbox, so that the available space in the panel is not enough to fit them all, Iconbox extends the panel beyond the screen edges. It also moves any panel plugin after Iconbox out of the screen and makes them unusable. This happens regardless of position or orientation of panel.
Devel branch has been merged in master. A 4.7.0 release will follow soon. If you think this bug is not fixed? Feel free to reopen the bug.
Move bug to Window Buttons; the merge of tasklist and iconbox.