1 files changed, 25 insertions, 2 deletions
diff --git a/src/widgets/util/qsystemtrayicon_x11.cpp b/src/widgets/util/qsystemtrayicon_x11.cpp
index eee100d279..ba80734876 100644
--- a/src/widgets/util/qsystemtrayicon_x11.cpp
+++ b/src/widgets/util/qsystemtrayicon_x11.cpp
@@ -107,13 +107,29 @@ QSystemTrayIconSys::QSystemTrayIconSys(QSystemTrayIcon *qIn)
     // If we have a visual that has an alpha channel, we can paint this widget with a transparent
     // background and it will work.
     // However, if there's no alpha channel visual, in order for transparent tray icons to work,
-    // we do not have a transparent background on the widget, but call xcb_clear_region before
-    // painting the icon
+    // we do not have a transparent background on the widget, but set the BackPixmap property of our
+    // window to ParentRelative (so that it inherits the background of its X11 parent window), call
+    // xcb_clear_region before painting (so that the inherited background is visible) and then grab
+    // the just-drawn background from the X11 server.
     bool hasAlphaChannel = false;
     QMetaObject::invokeMethod(QGuiApplication::platformNativeInterface(),
                               "systrayVisualHasAlphaChannel", Qt::DirectConnection,
                               Q_RETURN_ARG(bool, hasAlphaChannel));
     setAttribute(Qt::WA_TranslucentBackground, hasAlphaChannel);
+    if (!hasAlphaChannel) {
+        createWinId();
+        QMetaObject::invokeMethod(QGuiApplication::platformNativeInterface(),
+                                    "setParentRelativeBackPixmap", Qt::DirectConnection,
+                                    Q_ARG(const QWindow *, windowHandle())
+                                 );
+
+        // XXX: This is actually required, but breaks things ("QWidget::paintEngine: Should no
+        // longer be called"). Why is this needed? When the widget is drawn, we use tricks to grab
+        // the tray icon's background from the server. If the tray icon isn't visible (because
+        // another window is on top of it), the trick fails and instead uses the content of that
+        // other window as the background.
+        // setAttribute(Qt::WA_PaintOnScreen);
+    }
 
     addToTray();
 }
@@ -214,11 +230,18 @@ void QSystemTrayIconSys::paintEvent(QPaintEvent *)
         painter.setCompositionMode(QPainter::CompositionMode_Source);
         painter.fillRect(rect, Qt::transparent);
     } else {
+        // Without Qt::WA_TranslucentBackground, we use a ParentRelative BackPixmap and jump through
+        // some hops to draw this background below our icon. This clears the whole tray icon to its
+        // background color and thus causes flickering (you can see that the icon is being
+        // repainted). However, we can't really do much about this.
         QMetaObject::invokeMethod(QGuiApplication::platformNativeInterface(),
                                     "clearRegion", Qt::DirectConnection,
                                     Q_ARG(const QWindow *, windowHandle()),
                                     Q_ARG(const QRect&, rect)
                                  );
+        painter.drawPixmap(QPoint(0, 0),
+                        QGuiApplication::primaryScreen()->grabWindow(winId(),
+                                0, 0, rect.size().width(), rect.size().height()));
     }
     painter.setCompositionMode(QPainter::CompositionMode_SourceOver);
     q->icon().paint(&painter, rect);