issue has been caught in debian 8 ( gnome 3.14 ) and debian 9 ( gnome 3.22.2 )

Test case:

1. run app
2. summon notification area ( super + M )
3. click right button on icon in tray ( the menu will appear )
4. click left button on desktop to lose focus ( the menu will disappear  and notification area to )
5. summon notification area again and click right button on icon in tray ( nothing will happen ) 

QSystemTrayIconSys catch MouseButtonPress, ContextMenu first time and does not second so this code doesn't invoke:

void QSystemTrayIconSys::mousePressEvent(QMouseEvent *ev)
{
 QPoint globalPos = ev->globalPos();
#ifndef QT_NO_CONTEXTMENU
 if (ev->button() == Qt::RightButton && q->contextMenu())
 q->contextMenu()->popup(globalPos);
#endif
...