QSystemTrayIcon/X11: Create tray icon window when system tray appears

... and destroy it otherwise.

Fixes: QTBUG-61898
Fixes: QTBUG-73459
Done-with: Gatis Paeglis <gatis.paeglis@qt.io>
Change-Id: I6bd8f397f7ccdb123f6a60d4fa466f7b0d760dfc
Reviewed-by: Gatis Paeglis <gatis.paeglis@qt.io>

src/widgets/util/qsystemtrayicon_p.h

@@ -69,6 +69,7 @@
 QT_BEGIN_NAMESPACE
 
 class QSystemTrayIconSys;
+class QSystemTrayWatcher;
 class QPlatformSystemTrayIcon;
 class QToolButton;
 class QLabel;
@@ -90,6 +91,8 @@ public:
     void showMessage_sys(const QString &title, const QString &msg, const QIcon &icon,
                          QSystemTrayIcon::MessageIcon msgIcon, int msecs);
 
+    void destroyIcon();
+
     static bool isSystemTrayAvailable_sys();
     static bool supportsMessages_sys();
 
@@ -101,6 +104,7 @@ public:
     QSystemTrayIconSys *sys;
     QPlatformSystemTrayIcon *qpa_sys;
     bool visible;
+    QSystemTrayWatcher *trayWatcher;
 
 private:
     void install_sys_qpa();

src/widgets/util/qsystemtrayicon_x11.cpp

@@ -92,9 +92,6 @@ protected:
     virtual void resizeEvent(QResizeEvent *) override;
     virtual void moveEvent(QMoveEvent *) override;
 
-private slots:
-    void systemTrayWindowChanged(QScreen *screen);
-
 private:
     QSystemTrayIcon *q;
 };
@@ -116,15 +113,6 @@ QSystemTrayIconSys::QSystemTrayIconSys(QSystemTrayIcon *qIn)
     setMouseTracking(true);
 }
 
-void QSystemTrayIconSys::systemTrayWindowChanged(QScreen *)
-{
-    if (!locateSystemTray()) {
-        QBalloonTip::hideBalloon();
-        hide(); // still no luck
-        destroy();
-    }
-}
-
 QRect QSystemTrayIconSys::globalGeometry() const
 {
     return QRect(mapToGlobal(QPoint(0, 0)), size());
@@ -199,10 +187,41 @@ void QSystemTrayIconSys::resizeEvent(QResizeEvent *event)
 }
 ////////////////////////////////////////////////////////////////////////////
 
+class QSystemTrayWatcher: public QObject
+{
+    Q_OBJECT
+public:
+    QSystemTrayWatcher(QSystemTrayIcon *trayIcon)
+        : QObject(trayIcon)
+        , mTrayIcon(trayIcon)
+    {
+        // This code uses string-based syntax because we want to connect to a signal
+        // which is defined in XCB plugin - QXcbNativeInterface::systemTrayWindowChanged().
+        connect(qGuiApp->platformNativeInterface(), SIGNAL(systemTrayWindowChanged(QScreen*)),
+                this, SLOT(systemTrayWindowChanged(QScreen*)));
+    }
+
+private slots:
+    void systemTrayWindowChanged(QScreen *)
+    {
+        auto icon = static_cast<QSystemTrayIconPrivate *>(QObjectPrivate::get(mTrayIcon));
+        icon->destroyIcon();
+        if (icon->visible && locateSystemTray()) {
+            icon->sys = new QSystemTrayIconSys(mTrayIcon);
+            icon->sys->show();
+        }
+    }
+
+private:
+    QSystemTrayIcon *mTrayIcon = nullptr;
+};
+////////////////////////////////////////////////////////////////////////////
+
 QSystemTrayIconPrivate::QSystemTrayIconPrivate()
     : sys(0),
       qpa_sys(QGuiApplicationPrivate::platformTheme()->createPlatformSystemTrayIcon()),
-      visible(false)
+      visible(false),
+      trayWatcher(nullptr)
 {
 }
 
@@ -213,16 +232,21 @@ QSystemTrayIconPrivate::~QSystemTrayIconPrivate()
 
 void QSystemTrayIconPrivate::install_sys()
 {
+    Q_Q(QSystemTrayIcon);
+
     if (qpa_sys) {
         install_sys_qpa();
         return;
     }
-    Q_Q(QSystemTrayIcon);
-    if (!sys && locateSystemTray()) {
-        sys = new QSystemTrayIconSys(q);
-        QObject::connect(QGuiApplication::platformNativeInterface(), SIGNAL(systemTrayWindowChanged(QScreen*)),
-                         sys, SLOT(systemTrayWindowChanged(QScreen*)));
-        sys->show();
+
+    if (!sys) {
+        if (!trayWatcher)
+            trayWatcher = new QSystemTrayWatcher(q);
+
+        if (locateSystemTray()) {
+            sys = new QSystemTrayIconSys(q);
+            sys->show();
+        }
     }
 }
 
@@ -241,14 +265,21 @@ void QSystemTrayIconPrivate::remove_sys()
         remove_sys_qpa();
         return;
     }
+
+    destroyIcon();
+}
+
+void QSystemTrayIconPrivate::destroyIcon()
+{
     if (!sys)
         return;
     QBalloonTip::hideBalloon();
-    sys->hide(); // this should do the trick, but...
-    delete sys; // wm may resize system tray only for DestroyEvents
-    sys = 0;
+    sys->hide();
+    delete sys;
+    sys = nullptr;
 }
 
+
 void QSystemTrayIconPrivate::updateIcon_sys()
 {
     if (qpa_sys) {