Support transparency in qt5 system tray icons, regardless of tray visual

This ports the Qt4 method of supporting the system tray
window visual that has an alpha channel as well as ones
that do not. We detect whether or not we have a 32-bit
format and either use a Qt::transparent background, or
call xcb_clear_region before painting the icon.

[ChangeLog][Linux/XCB] Fix transparency of tray icons in cases where
there is no alpha channel or system tray visual.

Task-number: QTBUG-35832
Change-Id: I43d500c39846d2916dd39f8c47c8d85e59b49cae
Reviewed-by: Uli Schlachter <psychon@znc.in>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@digia.com>

1 files changed, 27 insertions, 5 deletions

diff --git a/src/widgets/util/qsystemtrayicon_x11.cpp b/src/widgets/util/qsystemtrayicon_x11.cpp
index 06eaf86004..e7007e4091 100644
--- a/src/widgets/util/qsystemtrayicon_x11.cpp
+++ b/src/widgets/util/qsystemtrayicon_x11.cpp
@@ -104,11 +104,24 @@ QSystemTrayIconSys::QSystemTrayIconSys(QSystemTrayIcon *qIn)
     setObjectName(QStringLiteral("QSystemTrayIconSys"));
     setToolTip(q->toolTip());
     setAttribute(Qt::WA_AlwaysShowToolTips, true);
-    setAttribute(Qt::WA_TranslucentBackground, true);
     setAttribute(Qt::WA_QuitOnClose, false);
     const QSize size(22, 22); // Gnome, standard size
     setGeometry(QRect(QPoint(0, 0), size));
     setMinimumSize(size);
+
+    // We need two different behaviors depending on whether the X11 visual for the system tray
+    // (a) exists and (b) supports an alpha channel, i.e. is 32 bits.
+    // 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
+    bool hasAlphaChannel = false;
+    QMetaObject::invokeMethod(QGuiApplication::platformNativeInterface(),
+                              "systrayVisualHasAlphaChannel", Qt::DirectConnection,
+                              Q_RETURN_ARG(bool, hasAlphaChannel));
+    setAttribute(Qt::WA_TranslucentBackground, hasAlphaChannel);
+
     addToTray();
 }
 
@@ -199,12 +212,21 @@ bool QSystemTrayIconSys::event(QEvent *e)
 
 void QSystemTrayIconSys::paintEvent(QPaintEvent *)
 {
-    // Note: Transparent pixels require a particular Visual which XCB
-    // currently does not support yet.
     const QRect rect(QPoint(0, 0), geometry().size());
     QPainter painter(this);
-    painter.setCompositionMode(QPainter::CompositionMode_Source);
-    painter.fillRect(rect, Qt::transparent);
+
+    // If we have Qt::WA_TranslucentBackground set, during widget creation
+    // we detected the systray visual supported an alpha channel
+    if (testAttribute(Qt::WA_TranslucentBackground)) {
+        painter.setCompositionMode(QPainter::CompositionMode_Source);
+        painter.fillRect(rect, Qt::transparent);
+    } else {
+        QMetaObject::invokeMethod(QGuiApplication::platformNativeInterface(),
+                                    "clearRegion", Qt::DirectConnection,
+                                    Q_ARG(const QWindow *, windowHandle()),
+                                    Q_ARG(const QRect&, rect)
+                                 );
+    }
     painter.setCompositionMode(QPainter::CompositionMode_SourceOver);
     q->icon().paint(&painter, rect);
 }