xcb: QSystemTrayIcon: grab background pixmap for compositing only once

Followup to 2203d9d93e24e00d6e9bc9bda0e65a0c7f9923cc: clearRegion takes
a little time, so doing grabWindow() too soon afterwards can result in
grabbing a pixmap with undesired leftovers.  And doing it every time
we render the icon causes flicker.  So, do clearRegion() ASAP when showing
the icon, wait for it to happen, then grabWindow() only the first time we
render, and reuse the resulting pixmap for all future calls to paintEvent().
(The downside is, if there is any corruption during the first grabWindow(),
it's going to stay there as long as the app is running.)

Task-number: QTBUG-35658
Change-Id: If881ab192dba43758dcbb0d080663ff42057fa4f
Reviewed-by: Jørgen Lind <jorgen.lind@theqtcompany.com>

3 files changed, 5 insertions, 11 deletions

diff --git a/src/plugins/platforms/xcb/qxcbnativeinterface.cpp b/src/plugins/platforms/xcb/qxcbnativeinterface.cpp
index 7d950f8dee..14f5cd0747 100644
--- a/src/plugins/platforms/xcb/qxcbnativeinterface.cpp
+++ b/src/plugins/platforms/xcb/qxcbnativeinterface.cpp
@@ -191,16 +191,6 @@ bool QXcbNativeInterface::systrayVisualHasAlphaChannel() {
     }
 }
 
-void QXcbNativeInterface::clearRegion(const QWindow *qwindow, const QRect& rect)
-{
-    if (const QPlatformWindow *platformWindow = qwindow->handle()) {
-        const QXcbWindow *qxwindow = static_cast<const QXcbWindow *>(platformWindow);
-        xcb_connection_t *xcb_conn = qxwindow->xcb_connection();
-
-        xcb_clear_area(xcb_conn, false, qxwindow->xcb_window(), rect.x(), rect.y(), rect.width(), rect.height());
-    }
-}
-
 void QXcbNativeInterface::setParentRelativeBackPixmap(const QWindow *qwindow)
 {
     if (const QPlatformWindow *platformWindow = qwindow->handle()) {
diff --git a/src/plugins/platforms/xcb/qxcbnativeinterface.h b/src/plugins/platforms/xcb/qxcbnativeinterface.h
index 742984ae36..388d774853 100644
--- a/src/plugins/platforms/xcb/qxcbnativeinterface.h
+++ b/src/plugins/platforms/xcb/qxcbnativeinterface.h
@@ -104,7 +104,6 @@ public:
 
     Q_INVOKABLE void beep();
     Q_INVOKABLE bool systemTrayAvailable(const QScreen *screen) const;
-    Q_INVOKABLE void clearRegion(const QWindow *qwindow, const QRect& rect);
     Q_INVOKABLE void setParentRelativeBackPixmap(const QWindow *window);
     Q_INVOKABLE bool systrayVisualHasAlphaChannel();
     Q_INVOKABLE bool requestSystemTrayWindowDock(const QWindow *window);
diff --git a/src/plugins/platforms/xcb/qxcbwindow.cpp b/src/plugins/platforms/xcb/qxcbwindow.cpp
index 9298e874b8..34289fb90e 100644
--- a/src/plugins/platforms/xcb/qxcbwindow.cpp
+++ b/src/plugins/platforms/xcb/qxcbwindow.cpp
@@ -2307,6 +2307,11 @@ void QXcbWindow::handleXEmbedMessage(const xcb_client_message_event_t *event)
     case XEMBED_EMBEDDED_NOTIFY:
         Q_XCB_CALL(xcb_map_window(xcb_connection(), m_window));
         xcbScreen()->windowShown(this);
+        // Without Qt::WA_TranslucentBackground, we use a ParentRelative BackPixmap.
+        // Clear the whole tray icon window to its background color as early as possible
+        // so that we can get a clean result from grabWindow() later.
+        Q_XCB_CALL(xcb_clear_area(xcb_connection(), false, m_window, 0, 0, geometry().width(), geometry().height()));
+        xcb_flush(xcb_connection());
         break;
     case XEMBED_FOCUS_IN:
         Qt::FocusReason reason;