Change to enter/leave policy while grabbing.

Sending enter and leave events to other windows than the grabbing
window is not logical. The policy should be that only the grabbing
window receives enter and leave events.

Changed the documentation accordingly and provided the necessary
changes to Windows implementation.

Also removed explicit leave event generation for widgets when
popup is opened as that is now redundant.

tst_QWidget::underMouse() test was changed to behave according to
new logic.

Task-number: QTBUG-27871
Change-Id: I127fb8685b4a4206d1a319f42cba491ec02bc8ca
Reviewed-by: Oliver Wolff <oliver.wolff@digia.com>
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@digia.com>

4 files changed, 61 insertions, 44 deletions

diff --git a/src/gui/kernel/qplatformwindow.cpp b/src/gui/kernel/qplatformwindow.cpp
index 2bc6b01b3e..edd9a3d436 100644
--- a/src/gui/kernel/qplatformwindow.cpp
+++ b/src/gui/kernel/qplatformwindow.cpp
@@ -466,12 +466,18 @@ bool QPlatformWindow::frameStrutEventsEnabled() const
         \li Mouse grab: Qt expects windows to automatically grab the mouse if the user presses
             a button until the button is released.
             Automatic grab should be released if some window is explicitly grabbed.
-        \li Enter/Leave events: Enter and leave events should be sent independently of
-            explicit mouse grabs (\c{setMouseGrabEnabled()}). That is, if the mouse leaves
-            a window that has explicit mouse grab, a leave event should be sent and other
-            windows should get enter/leave events as well as the mouse traverses them.
-            For automatic mouse grab, however, a leave event should be sent when the
-            button is released.
+        \li Enter/Leave events: If there is a window explicitly grabbing mouse events
+            (\c{setMouseGrabEnabled()}), enter and leave events should only be sent to the
+            grabbing window when mouse cursor passes over the grabbing window boundary.
+            Other windows will not receive enter or leave events while the grab is active.
+            While an automatic mouse grab caused by a mouse button press is active, no window
+            will receive enter or leave events. When the last mouse button is released, the
+            autograbbing window will receive leave event if mouse cursor is no longer within
+            the window boundary.
+            When any grab starts, the window under cursor will receive a leave event unless
+            it is the grabbing window.
+            When any grab ends, the window under cursor will receive an enter event unless it
+            was the grabbing window.
         \li Window positioning: When calling \c{QWindow::setFramePos()}, the flag
             \c{QWindowPrivate::positionPolicy} is set to \c{QWindowPrivate::WindowFrameInclusive}.
             This means the position includes the window frame, whose size is at this point
diff --git a/src/plugins/platforms/windows/qwindowsmousehandler.cpp b/src/plugins/platforms/windows/qwindowsmousehandler.cpp
index 2f624c3e16..2f2d5d2c59 100644
--- a/src/plugins/platforms/windows/qwindowsmousehandler.cpp
+++ b/src/plugins/platforms/windows/qwindowsmousehandler.cpp
@@ -130,7 +130,8 @@ QWindowsMouseHandler::QWindowsMouseHandler() :
     m_windowUnderMouse(0),
     m_trackedWindow(0),
     m_touchDevice(0),
-    m_leftButtonDown(false)
+    m_leftButtonDown(false),
+    m_previousCaptureWindow(0)
 {
 }
 
@@ -212,6 +213,7 @@ bool QWindowsMouseHandler::translateMouseEvent(QWindow *window, HWND hwnd,
                 if (QWindowsContext::verboseEvents)
                     qDebug() << "Automatic mouse capture for missing buttondown event" << window;
             }
+            m_previousCaptureWindow = window;
             return true;
         } else if (m_leftButtonDown && !actualLeftDown) {
             m_leftButtonDown = false;
@@ -253,12 +255,13 @@ bool QWindowsMouseHandler::translateMouseEvent(QWindow *window, HWND hwnd,
         }
     }
 
+    const bool hasCapture = platformWindow->hasMouseCapture();
+    const bool currentNotCapturing = hasCapture && currentWindowUnderMouse != window;
 #ifndef Q_OS_WINCE
     // Enter new window: track to generate leave event.
-    // If there is an active capture, we must track the actual capture window instead of window
-    // under cursor or leaves will trigger constantly, so always track the window we got
-    // native mouse event for.
-    if (window != m_trackedWindow) {
+    // If there is an active capture, only track if the current window is capturing,
+    // so we don't get extra leave when cursor leaves the application.
+    if (window != m_trackedWindow && !currentNotCapturing) {
         TRACKMOUSEEVENT tme;
         tme.cbSize = sizeof(TRACKMOUSEEVENT);
         tme.dwFlags = TME_LEAVE;
@@ -270,39 +273,53 @@ bool QWindowsMouseHandler::translateMouseEvent(QWindow *window, HWND hwnd,
     }
 #endif // !Q_OS_WINCE
 
-    // Qt expects enter/leave events for windows even when some window is capturing mouse input,
-    // except for automatic capture when mouse button is pressed - in that case enter/leave
-    // should be sent only after the last button is released.
-    // We need to track m_windowUnderMouse separately from m_trackedWindow, as
-    // Windows mouse tracking will not trigger WM_MOUSELEAVE for leaving window when
-    // mouse capture is set.
-    if (!platformWindow->hasMouseCapture()
-        || !platformWindow->testFlag(QWindowsWindow::AutoMouseCapture)) {
-        if (m_windowUnderMouse != currentWindowUnderMouse) {
-            if (m_windowUnderMouse) {
-                if (QWindowsContext::verboseEvents)
-                    qDebug() << "Synthetic leave for " << m_windowUnderMouse;
-                QWindowSystemInterface::handleLeaveEvent(m_windowUnderMouse);
-                // Clear tracking if we are no longer over application,
-                // since we have already sent the leave.
-                if (!currentWindowUnderMouse)
-                    m_trackedWindow = 0;
-            }
-
-            if (currentWindowUnderMouse) {
-                if (QWindowsContext::verboseEvents)
-                    qDebug() << "Entering " << currentWindowUnderMouse;
-                QWindowsWindow::baseWindowOf(currentWindowUnderMouse)->applyCursor();
-                QWindowSystemInterface::handleEnterEvent(currentWindowUnderMouse,
-                                                         currentWindowUnderMouse->mapFromGlobal(globalPosition),
-                                                         globalPosition);
+    // No enter or leave events are sent as long as there is an autocapturing window.
+    if (!hasCapture || !platformWindow->testFlag(QWindowsWindow::AutoMouseCapture)) {
+        // Leave is needed if:
+        // 1) There is no capture and we move from a window to another window.
+        //    Note: Leaving the application entirely is handled in WM_MOUSELEAVE case.
+        // 2) There is capture and we move out of the capturing window.
+        // 3) There is a new capture and we were over another window.
+        if ((m_windowUnderMouse && m_windowUnderMouse != currentWindowUnderMouse
+                && (!hasCapture || window == m_windowUnderMouse))
+            || (hasCapture && m_previousCaptureWindow != window && m_windowUnderMouse
+                && m_windowUnderMouse != window)) {
+            if (QWindowsContext::verboseEvents)
+                qDebug() << "Synthetic leave for " << m_windowUnderMouse;
+            QWindowSystemInterface::handleLeaveEvent(m_windowUnderMouse);
+            if (currentNotCapturing) {
+                // Clear tracking if capturing and current window is not the capturing window
+                // to avoid leave when mouse actually leaves the application.
+                m_trackedWindow = 0;
+                // We are not officially in any window, but we need to set some cursor to clear
+                // whatever cursor the left window had, so apply the cursor of the capture window.
+                QWindowsWindow::baseWindowOf(window)->applyCursor();
             }
         }
+        // Enter is needed if:
+        // 1) There is no capture and we move to a new window.
+        // 2) There is capture and we move into the capturing window.
+        // 3) The capture just ended and we are over non-capturing window.
+        if ((currentWindowUnderMouse && m_windowUnderMouse != currentWindowUnderMouse
+                && (!hasCapture || currentWindowUnderMouse == window))
+            || (m_previousCaptureWindow && window != m_previousCaptureWindow && currentWindowUnderMouse
+                && currentWindowUnderMouse != m_previousCaptureWindow)) {
+            if (QWindowsContext::verboseEvents)
+                qDebug() << "Entering " << currentWindowUnderMouse;
+            QWindowsWindow::baseWindowOf(currentWindowUnderMouse)->applyCursor();
+            QWindowSystemInterface::handleEnterEvent(currentWindowUnderMouse,
+                                                     currentWindowUnderMouse->mapFromGlobal(globalPosition),
+                                                     globalPosition);
+        }
+        // We need to track m_windowUnderMouse separately from m_trackedWindow, as
+        // Windows mouse tracking will not trigger WM_MOUSELEAVE for leaving window when
+        // mouse capture is set.
         m_windowUnderMouse = currentWindowUnderMouse;
     }
 
     QWindowSystemInterface::handleMouseEvent(window, winEventPosition, globalPosition, buttons,
                                              QWindowsKeyMapper::queryKeyboardModifiers());
+    m_previousCaptureWindow = hasCapture ? window : 0;
     return true;
 }
 
diff --git a/src/plugins/platforms/windows/qwindowsmousehandler.h b/src/plugins/platforms/windows/qwindowsmousehandler.h
index e43d20ed6b..965deb4e0f 100644
--- a/src/plugins/platforms/windows/qwindowsmousehandler.h
+++ b/src/plugins/platforms/windows/qwindowsmousehandler.h
@@ -82,6 +82,7 @@ private:
     QHash<DWORD, int> m_touchInputIDToTouchPointID;
     QTouchDevice *m_touchDevice;
     bool m_leftButtonDown;
+    QWindow *m_previousCaptureWindow;
 };
 
 Qt::MouseButtons QWindowsMouseHandler::keyStateToMouseButtons(int wParam)
diff --git a/src/widgets/kernel/qapplication_qpa.cpp b/src/widgets/kernel/qapplication_qpa.cpp
index e8db364473..21bc9506d4 100644
--- a/src/widgets/kernel/qapplication_qpa.cpp
+++ b/src/widgets/kernel/qapplication_qpa.cpp
@@ -250,13 +250,6 @@ void QApplicationPrivate::openPopup(QWidget *popup)
             QApplication::sendEvent(fw, &e);
         }
     }
-
-    // Dispatch leave for last mouse receiver to update undermouse states
-    if (qt_last_mouse_receiver && !QWidget::mouseGrabber()) {
-        QApplicationPrivate::dispatchEnterLeave(0, qt_last_mouse_receiver.data(),
-                                                QGuiApplicationPrivate::lastCursorPosition);
-        qt_last_mouse_receiver = 0;
-    }
 }
 
 void QApplicationPrivate::initializeMultitouch_sys()