Implement automatic mouse grabbing on mouse button press.

QWidget::grabMouse() documentation states that Qt automatically grabs
the mouse when a button is pressed and ungrabs it when it is released.
Implemented this functionality to make Qt5 behavior similar to Qt4.

Task-number: QTBUG-25977
Change-Id: I17aa7e8190f4f5224ebc382d62970c86ae492bdb
Reviewed-by: Samuel Rødal <samuel.rodal@nokia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@nokia.com>

src/widgets/kernel/qapplication.cpp

@@ -3153,6 +3153,8 @@ bool QApplication::notify(QObject *receiver, QEvent *e)
                     d->toolTipGlobalPos = mouse->globalPos();
                     d->toolTipWakeUp.start(d->toolTipFallAsleep.isActive()?20:700, this);
                 }
+
+                d->handleAutomaticMouseGrab(w, mouse);
             }
 
             bool eventAccepted = mouse->isAccepted();

src/widgets/kernel/qapplication_p.h

@@ -167,6 +167,7 @@ public:
     void openPopup(QWidget *popup);
     static void setFocusWidget(QWidget *focus, Qt::FocusReason reason);
     static QWidget *focusNextPrevChild_helper(QWidget *toplevel, bool next);
+    void handleAutomaticMouseGrab(QWidget *widget, QMouseEvent *e);
 
 #ifndef QT_NO_SESSIONMANAGER
     QSessionManager *session_manager;

src/widgets/kernel/qapplication_qpa.cpp

@@ -74,7 +74,8 @@ QT_BEGIN_NAMESPACE
 
 static QString appName;
 static QString appFont;
-static bool popupGrabOk;
+static bool popupGrabOk = false;
+static QPointer<QWidget> autoGrabber;
 extern QWidget *qt_button_down;
 extern QWidget *qt_popup_down;
 extern bool qt_replay_popup_mouse_event;
@@ -148,6 +149,28 @@ void QApplicationPrivate::notifyActiveWindowChange(QWindow *previous)
     QApplication::setActiveWindow(tlw);
 }
 
+void QApplicationPrivate::handleAutomaticMouseGrab(QWidget *widget, QMouseEvent *e)
+{
+    // Grab the mouse automatically for current window when any button is pressed,
+    // unless there is an active mousegrabber or mouse is being grabbed for a popup.
+    if (e->type() == QEvent::MouseButtonPress || e->type() == QEvent::MouseButtonRelease) {
+        if (e->buttons() == Qt::NoButton) {
+            // No buttons remain pressed, so release the grab unless grab has been acquired
+            // for some other reason in the meantime.
+            if (autoGrabber && !QWidget::mouseGrabber() && !popupGrabOk)
+                qt_widget_private(autoGrabber)->stealMouseGrab(false);
+            autoGrabber = 0;
+        } else {
+            // Some buttons are pressed, grab mouse input for current window,
+            // unless there is already an active grab.
+            if (!autoGrabber && !QWidget::mouseGrabber() && !popupGrabOk) {
+                autoGrabber = widget->window();
+                qt_widget_private(autoGrabber)->stealMouseGrab(true);
+            }
+        }
+    }
+}
+
 static void ungrabKeyboardForPopup(QWidget *popup)
 {
     if (QWidget::keyboardGrabber())
@@ -169,6 +192,9 @@ static void grabForPopup(QWidget *popup)
     Q_ASSERT(popup->testAttribute(Qt::WA_WState_Created));
     popupGrabOk = qt_widget_private(popup)->stealKeyboardGrab(true);
     if (popupGrabOk) {
+        if (autoGrabber)
+            qt_widget_private(autoGrabber)->stealMouseGrab(false);
+        autoGrabber = 0;
         popupGrabOk = qt_widget_private(popup)->stealMouseGrab(true);
         if (!popupGrabOk) {
             // transfer grab back to the keyboard grabber if any