Some items become missing from "Winows Buttons" area
The display contains a panel with the "Windows Buttons" item. A wine application, in my instance, PDF-XChange Viewer free version 2.0, can sometimes make itself and possibly other running windows disappear from the "Windows Buttons" area. All running windows are accessible via Alt-tab. This only happens with panel 2, at the bottom. I later created a "Windows Buttons" area at the top (panel 1). That panel shows correctly. Bringing up panel preferences for panel 2 immediately results is all windows being present again.