Windows QPA: Detect screen by mouse position when dragging a window

When dragging a window by its border, detect the screen by mouse position
to prevent it from oscillating between screens when it resizes.

Task-number: QTBUG-62971
Change-Id: I0a4a584ef8ff3bb7288d1abec4de51fb4091dccd
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>
Reviewed-by: Thorbjørn Lund Martsum <tmartsum@gmail.com>

src/plugins/platforms/windows/qtwindowsglobal.h

@@ -101,6 +101,8 @@ enum WindowsEventType // Simplify event types
     FocusOutEvent = WindowEventFlag + 18,
     WhatsThisEvent = WindowEventFlag + 19,
     DpiChangedEvent = WindowEventFlag + 21,
+    EnterSizeMoveEvent = WindowEventFlag + 22,
+    ExitSizeMoveEvent = WindowEventFlag + 23,
     MouseEvent = MouseEventFlag + 1,
     MouseWheelEvent = MouseEventFlag + 2,
     CursorEvent = MouseEventFlag + 3,
@@ -274,6 +276,10 @@ inline QtWindows::WindowsEventType windowsEventType(UINT message, WPARAM wParamI
         return QtWindows::DeviceChangeEvent;
     case WM_DPICHANGED:
         return QtWindows::DpiChangedEvent;
+    case WM_ENTERSIZEMOVE:
+        return QtWindows::EnterSizeMoveEvent;
+    case WM_EXITSIZEMOVE:
+        return QtWindows::ExitSizeMoveEvent;
     default:
         break;
     }

src/plugins/platforms/windows/qwindowscontext.cpp

@@ -1055,6 +1055,13 @@ bool QWindowsContext::windowsProc(HWND hwnd, UINT message,
             return d->m_mouseHandler.translateMouseEvent(platformWindow->window(), hwnd, et, msg, result);
 #endif
         break;
+    case QtWindows::EnterSizeMoveEvent:
+        platformWindow->setFlag(QWindowsWindow::ResizeMoveActive);
+        return true;
+    case QtWindows::ExitSizeMoveEvent:
+        platformWindow->clearFlag(QWindowsWindow::ResizeMoveActive);
+        platformWindow->checkForScreenChanged();
+        return true;
     case QtWindows::ScrollEvent:
 #if QT_CONFIG(sessionmanager)
         return platformSessionManager()->isInteractionBlocked() ? true : d->m_mouseHandler.translateScrollEvent(platformWindow->window(), hwnd, msg, result);

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -1527,6 +1527,26 @@ void QWindowsWindow::handleResized(int wParam)
     }
 }
 
+void QWindowsWindow::checkForScreenChanged()
+{
+    if (parent())
+        return;
+
+    QPlatformScreen *currentScreen = screen();
+    const auto &screenManager = QWindowsContext::instance()->screenManager();
+    // QTBUG-62971: When dragging a window by its border, detect by mouse position
+    // to prevent it from oscillating between screens when it resizes
+    const QWindowsScreen *newScreen = testFlag(ResizeMoveActive)
+        ? screenManager.screenAtDp(QWindowsCursor::mousePosition())
+        : screenManager.screenForHwnd(m_data.hwnd);
+    if (newScreen != nullptr && newScreen != currentScreen) {
+        qCDebug(lcQpaWindows).noquote().nospace() << __FUNCTION__
+            << ' ' << window() << " \"" << currentScreen->name()
+            << "\"->\"" << newScreen->name() << '"';
+        QWindowSystemInterface::handleWindowScreenChanged(window(), newScreen->screen());
+    }
+}
+
 void QWindowsWindow::handleGeometryChange()
 {
     const QRect previousGeometry = m_data.geometry;
@@ -1541,17 +1561,9 @@ void QWindowsWindow::handleGeometryChange()
         && !(m_data.geometry.width() > previousGeometry.width() || m_data.geometry.height() > previousGeometry.height())) {
         fireExpose(QRect(QPoint(0, 0), m_data.geometry.size()), true);
     }
-    if (!parent() && previousGeometry.topLeft() != m_data.geometry.topLeft()) {
-        QPlatformScreen *currentScreen = screen();
-        const QWindowsScreen *newScreen =
-            QWindowsContext::instance()->screenManager().screenForHwnd(m_data.hwnd);
-        if (newScreen != nullptr && newScreen != currentScreen) {
-            qCDebug(lcQpaWindows).noquote().nospace() << __FUNCTION__
-                << ' ' << window() << " \"" << currentScreen->name()
-                << "\"->\"" << newScreen->name() << '"';
-            QWindowSystemInterface::handleWindowScreenChanged(window(), newScreen->screen());
-        }
-    }
+
+    checkForScreenChanged();
+
     if (testFlag(SynchronousGeometryChangeEvent))
         QWindowSystemInterface::flushWindowSystemEvents(QEventLoop::ExcludeUserInputEvents);
 

src/plugins/platforms/windows/qwindowswindow.h

@@ -208,6 +208,7 @@ public:
         Compositing = 0x200000,
         HasBorderInFullScreen = 0x400000,
         WithinDpiChanged = 0x800000,
+        ResizeMoveActive = 0x2000000
     };
 
     QWindowsWindow(QWindow *window, const QWindowsWindowData &data);
@@ -313,6 +314,8 @@ public:
     void alertWindow(int durationMs = 0);
     void stopAlertWindow();
 
+    void checkForScreenChanged();
+
     static void setTouchWindowTouchTypeStatic(QWindow *window, QWindowsWindowFunctions::TouchWindowTouchTypes touchTypes);
     void registerTouchWindow(QWindowsWindowFunctions::TouchWindowTouchTypes touchTypes = QWindowsWindowFunctions::NormalTouch);
     static void setHasBorderInFullScreenStatic(QWindow *window, bool border);