It occurs to me that this might be quite a corner-case because my monitor 2 is not a tall as monitor 1, so it might be calculating the margin from the bounding box of the two monitors together rather than from the edge of the visible area where the panel is actually located.
- Bug#823946: lxpanel: windows maximize behind panel on second ... Tom Vajzovic
- Bug#823946: Acknowledgement (lxpanel: windows maximize b... Tom Vajzovic