use the whole geometry for the struts

screen()->virtualSize() is the union of the sizes
of all screens
screen()->geometry().size() is the size of just the
single screen

shell/panelview.cpp

@@ -856,7 +856,7 @@ void PanelView::updateStruts()
     if (m_visibilityMode == NormalPanel) {
         const QRect thisScreen = screen()->geometry();
         // QScreen::virtualGeometry() is very unreliable (Qt 5.5)
-        const QRect wholeScreen = QRect(QPoint(0, 0), screen()->geometry().size());
+        const QRect wholeScreen = QRect(QPoint(0, 0), screen()->virtualSize());
 
         //Extended struts against a screen edge near to another screen are really harmful, so windows maximized under the panel is a lesser pain
         //TODO: force "windows can cover" in those cases?