When I mouse over a panel with "Don't reserve space on borders" set and I hover over one of the "window buttons", xfwm4 (I guess) draws a border around the window frame. When a window is placed such that its titlebar is under said panel, this border does not respect the layering and appears over the top of the window buttons. Thus, when the mouse hovers over the border, it is no longer hovering over the icon, switching the border off. Of course, with the border no longer there, it registers as hovering over the icon again, switching it back on. Practically, this causes a spastic flicker until the mouse is moved. It's only mildly annoying. Possible solution: draw the border in the same layer (z-axis) as the window, thus resolving the conflict by putting the border under the panel.
The panel itself draws a frame for window buttons, the window manager is not involved there.
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