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>

2 files changed, 49 insertions, 31 deletions

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)