QmlDesigner: Fix drag and drop

The release mouse event does not come through if we start a drag.
There was already a workaround that stopped working with 5.8 and
did rely on private API.
I removed all usage of private API and added a simple workaround
that synthesises a mouse release event.

The actual execution of the drag is now asynchronous to
ensure the release event is delivered properly.

I removed all dependencies on private API in the designer.

In Qt 5.9 the issue seems to be fixed in Qt.
The workaround does not seem to get in the way of the fix.
Eventually, the workaround can be removed.

Change-Id: I9b45b255da5e44c26aba2acf4a42f88537126f75
Reviewed-by: Tim Jenssen <tim.jenssen@qt.io>

share/qtcreator/qmldesigner/itemLibraryQmlSources/ItemDelegate.qml

@@ -73,12 +73,8 @@ Item {
             id: mouseRegion
             anchors.fill: parent
 
-            property bool reallyPressed: false
-            property int pressedX
-            property int pressedY
-
             onPressed: {
-                rootView.startDragAndDrop(itemLibraryEntry)
+                rootView.startDragAndDrop(mouseRegion, itemLibraryEntry)
             }
         }
     }

src/plugins/qmldesigner/components/itemlibrary/itemlibrarywidget.cpp

@@ -48,6 +48,7 @@
 #include <QTabBar>
 #include <QImageReader>
 #include <QMimeData>
+#include <QMouseEvent>
 #include <QWheelEvent>
 #include <QMenu>
 #include <QApplication>
@@ -55,10 +56,6 @@
 #include <QShortcut>
 #include <QQuickItem>
 
-#include <private/qquickwidget_p.h> // mouse ungrabbing workaround on quickitems
-#include <private/qquickwindow_p.h> // mouse ungrabbing workaround on quickitems
-
-
 namespace QmlDesigner {
 
 ItemLibraryWidget::ItemLibraryWidget(QWidget *parent) :
@@ -265,14 +262,7 @@ void ItemLibraryWidget::setResourcePath(const QString &resourcePath)
     updateSearch();
 }
 
-static void ungrabMouseOnQMLWorldWorkAround(QQuickWidget *quickWidget)
-{
-    const QQuickWidgetPrivate *widgetPrivate = QQuickWidgetPrivate::get(quickWidget);
-    if (widgetPrivate && widgetPrivate->offscreenWindow && widgetPrivate->offscreenWindow->mouseGrabberItem())
-        widgetPrivate->offscreenWindow->mouseGrabberItem()->ungrabMouse();
-}
-
-void ItemLibraryWidget::startDragAndDrop(QVariant itemLibraryId)
+void ItemLibraryWidget::startDragAndDrop(QQuickItem *mouseArea, QVariant itemLibraryId)
 {
     m_currentitemLibraryEntry = itemLibraryId.value<ItemLibraryEntry>();
 
@@ -283,9 +273,14 @@ void ItemLibraryWidget::startDragAndDrop(QVariant itemLibraryId)
                         m_currentitemLibraryEntry.libraryEntryIconPath()));
     drag->setMimeData(mimeData);
 
-    drag->exec();
+    /* Workaround for bug in Qt. The release event is not delivered for Qt < 5.9 if a drag is started */
+    QMouseEvent event (QEvent::MouseButtonRelease, QPoint(-1, -1), Qt::LeftButton, Qt::LeftButton, Qt::NoModifier);
+    QApplication::sendEvent(mouseArea, &event);
 
-    ungrabMouseOnQMLWorldWorkAround(m_itemViewQuickWidget.data());
+    QTimer::singleShot(0, [drag]() {
+        drag->exec();
+        drag->deleteLater();
+    });
 }
 
 void ItemLibraryWidget::removeImport(const QString &name)

src/plugins/qmldesigner/components/itemlibrary/itemlibrarywidget.h

@@ -84,7 +84,7 @@ public:
 
     void setModel(Model *model);
 
-    Q_INVOKABLE void startDragAndDrop(QVariant itemLibId);
+    Q_INVOKABLE void startDragAndDrop(QQuickItem *mouseArea, QVariant itemLibId);
 
 protected:
     void removeImport(const QString &name);

src/plugins/qmldesigner/qmldesignerplugin.pro

 QT += quickwidgets
-QT += widgets-private quick-private quickwidgets-private core-private gui-private #mouse ungrabbing workaround on quickitems
 CONFIG += exceptions
 
 INCLUDEPATH += $$PWD