Windows QPA: Move transient children with a window move

When moving a window with keyboard shortcuts, popup menus currently do
not follow along. Allow this to happen by accounting for a window's
transient children, and moving them after the owning window has finished
its move.

Fixes: QTBUG-106483
Pick-to: 6.6 6.5
Change-Id: Id51a7c0163e4fdd2d139565f2bf500a3fc997488
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>

src/plugins/platforms/windows/qwindowswindow.cpp

@@ -937,6 +937,7 @@ QWindowsWindowData
 
     result.geometry = obtainedGeometry;
     result.restoreGeometry = frameGeometry(result.hwnd, topLevel);
+    result.preMoveGeometry = obtainedGeometry;
     result.fullFrameMargins = context->margins;
     result.embedded = embedded;
     result.hasFrame = hasFrame;
@@ -2153,11 +2154,56 @@ void QWindowsWindow::setGeometry(const QRect &rectIn)
     }
 }
 
+QWindow *QWindowsWindow::topTransientOf(QWindow *w)
+{
+    while (QWindow *transientParent = w->transientParent())
+        w = transientParent;
+    return w;
+}
+
+void QWindowsWindow::moveTransientChildren()
+{
+    // We need the topmost Transient parent since it is the window that will initiate
+    // the chain of moves, and is the only one with an already up to date DPI, which we
+    // need for the scaling.
+    const QWindowsWindow *topTransient = QWindowsWindow::windowsWindowOf(topTransientOf(window()));
+
+    const QWindow *currentWindow = window();
+    const QWindowList allWindows = QGuiApplication::allWindows();
+    for (QWindow *w : allWindows) {
+        if (w->transientParent() == currentWindow && w != currentWindow && w->isVisible()) {
+            QWindowsWindow *transientChild = QWindowsWindow::windowsWindowOf(w);
+
+            RECT oldChildPos{};
+            GetWindowRect(transientChild->handle(), &oldChildPos);
+            const RECT oldParentPos = RECTfromQRect(preMoveRect());
+
+            const qreal scale =
+                    QHighDpiScaling::roundScaleFactor(qreal(topTransient->savedDpi()) / QWindowsScreen::baseDpi) /
+                    QHighDpiScaling::roundScaleFactor(qreal(transientChild->savedDpi()) / QWindowsScreen::baseDpi);
+
+            const RECT offset =
+                    RECTfromQRect(QRect(scale * (oldChildPos.left - oldParentPos.left),
+                                        scale * (oldChildPos.top - oldParentPos.top), 0, 0));
+            const RECT newParentPos = RECTfromQRect(m_data.geometry);
+            const RECT newChildPos { newParentPos.left + offset.left,
+                                     newParentPos.top + offset.top,
+                                     transientChild->geometry().width(),
+                                     transientChild->geometry().height() };
+
+            SetWindowPos(transientChild->handle(), nullptr, newChildPos.left, newChildPos.top,
+                         newChildPos.right, newChildPos.bottom, SWP_NOZORDER | SWP_NOACTIVATE);
+        }
+    }
+}
+
 void QWindowsWindow::handleMoved()
 {
+    setPreMoveRect(geometry());
     // Minimize/Set parent can send nonsensical move events.
     if (!IsIconic(m_data.hwnd) && !testFlag(WithinSetParent))
         handleGeometryChange();
+    moveTransientChildren();
 }
 
 void QWindowsWindow::handleResized(int wParam, LPARAM lParam)

src/plugins/platforms/windows/qwindowswindow.h

@@ -78,6 +78,7 @@ struct QWindowsWindowData
 {
     Qt::WindowFlags flags;
     QRect geometry;
+    QRect preMoveGeometry;
     QRect restoreGeometry;
     QMargins fullFrameMargins; // Do not use directly for windows, see FrameDirty.
     QMargins customMargins;    // User-defined, additional frame for NCCALCSIZE
@@ -222,6 +223,11 @@ public:
     QRect restoreGeometry() const { return m_data.restoreGeometry; }
     void updateRestoreGeometry();
 
+    static QWindow *topTransientOf(QWindow *w);
+    QRect preMoveRect() const { return m_data.preMoveGeometry; }
+    void setPreMoveRect(const QRect &rect) { m_data.preMoveGeometry = rect; }
+    void moveTransientChildren();
+
     void setVisible(bool visible) override;
     bool isVisible() const;
     bool isExposed() const override { return testFlag(Exposed); }