Fix QWidget::setWindowOpacity() when called before show().

Pass opacity from the QWidget to QWindow and to the platform
windows.

Task-number: QTBUG-28477
Change-Id: If5a85d9183bd1ca33dac2052936ecd1e6c0b5f6c
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>

src/gui/kernel/qwindow.cpp

@@ -654,9 +654,11 @@ void QWindow::lower()
 void QWindow::setOpacity(qreal level)
 {
     Q_D(QWindow);
-    if (d->platformWindow) {
+    if (level == d->opacity) // #fixme: Add property for 5.1
+        return;
+    d->opacity = level;
+    if (d->platformWindow)
         d->platformWindow->setOpacity(level);
-    }
 }
 
 /*!

src/gui/kernel/qwindow_p.h

@@ -89,6 +89,7 @@ public:
         , receivedExpose(false)
         , positionPolicy(WindowFrameExclusive)
         , contentOrientation(Qt::PrimaryOrientation)
+        , opacity(qreal(1.0))
         , minimumSize(0, 0)
         , maximumSize(QWINDOWSIZE_MAX, QWINDOWSIZE_MAX)
         , modality(Qt::NonModal)
@@ -135,6 +136,7 @@ public:
     bool receivedExpose;
     PositionPolicy positionPolicy;
     Qt::ScreenOrientation contentOrientation;
+    qreal opacity;
 
     QSize minimumSize;
     QSize maximumSize;

src/plugins/platforms/cocoa/qcocoawindow.mm

@@ -49,6 +49,7 @@
 #include <QtCore/qfileinfo.h>
 #include <QtCore/private/qcore_mac_p.h>
 #include <qwindow.h>
+#include <private/qwindow_p.h>
 #include <qpa/qwindowsysteminterface.h>
 #include <qpa/qplatformscreen.h>
 
@@ -649,6 +650,10 @@ void QCocoaWindow::recreateWindow(const QPlatformWindow *parentWindow)
         NSRect frame = NSMakeRect(rect.x(), rect.y(), rect.width(), rect.height());
         [m_contentView setFrame:frame];
     }
+
+    const qreal opacity = qt_window_private(window())->opacity;
+    if (!qFuzzyCompare(opacity, qreal(1.0)))
+        setOpacity(opacity);
 }
 
 NSWindow * QCocoaWindow::createNSWindow()

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -731,6 +731,9 @@ QWindowsWindow::QWindowsWindow(QWindow *aWindow, const WindowData &data) :
     if (QWindowsContext::instance()->systemInfo() & QWindowsContext::SI_SupportsTouch)
         QWindowsContext::user32dll.registerTouchWindow(m_data.hwnd, 0);
     setWindowState(aWindow->windowState());
+    const qreal opacity = qt_window_private(aWindow)->opacity;
+    if (!qFuzzyCompare(opacity, qreal(1.0)))
+        setOpacity(opacity);
 }
 
 QWindowsWindow::~QWindowsWindow()

src/plugins/platforms/xcb/qxcbwindow.cpp

@@ -385,6 +385,10 @@ void QXcbWindow::create()
 #ifndef QT_NO_DRAGANDDROP
     connection()->drag()->dndEnable(this, true);
 #endif
+
+    const qreal opacity = qt_window_private(window())->opacity;
+    if (!qFuzzyCompare(opacity, qreal(1.0)))
+        setOpacity(opacity);
 }
 
 QXcbWindow::~QXcbWindow()

src/widgets/kernel/qwidget.cpp

@@ -10266,6 +10266,7 @@ void QWidget::setWindowOpacity(qreal opacity)
     QTLWExtra *extra = d->topData();
     extra->opacity = uint(opacity * 255);
     setAttribute(Qt::WA_WState_WindowOpacitySet);
+    d->setWindowOpacity_sys(opacity);
 
     if (!testAttribute(Qt::WA_WState_Created))
         return;
@@ -10280,8 +10281,6 @@ void QWidget::setWindowOpacity(qreal opacity)
         return;
     }
 #endif
-
-    d->setWindowOpacity_sys(opacity);
 }
 
 /*!