Dialog: Fall back to transient parent to make dialog window get centered

Unlike QWidget-based application, a dialog displayed in QML app doesn't
have QWidget parent, so it would be centered in the current screen.
Let's fall back to transient parent window if widgetParent() is missing.

It's tempting to rewrite adjustPosition() to not depend on QWidget at all,
but that seems not easy. Lookup path of window() and transientParentWindow()
is slightly different for example, and QWidget::pos() is used instead of
mapToGlobal({0, 0}) if the widget is embedded into a native window.

Fixes: QTBUG-63406
Change-Id: If72d90aee8d972240243184de4d3c09d77f704ff
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>

1 files changed, 14 insertions, 1 deletions

diff --git a/src/widgets/dialogs/qdialog.cpp b/src/widgets/dialogs/qdialog.cpp
index 52976611aa..c416ba9a5c 100644
--- a/src/widgets/dialogs/qdialog.cpp
+++ b/src/widgets/dialogs/qdialog.cpp
@@ -871,17 +871,25 @@ void QDialog::showEvent(QShowEvent *event)
 /*! \internal */
 void QDialog::adjustPosition(QWidget* w)
 {
+    Q_D(QDialog);
+
     if (const QPlatformTheme *theme = QGuiApplicationPrivate::platformTheme())
         if (theme->themeHint(QPlatformTheme::WindowAutoPlacement).toBool())
             return;
     QPoint p(0, 0);
     int extraw = 0, extrah = 0;
-    if (w)
+    const QWindow *parentWindow = nullptr;
+    if (w) {
         w = w->window();
+    } else {
+        parentWindow = d->transientParentWindow();
+    }
     QRect desk;
     QScreen *scrn = nullptr;
     if (w)
         scrn = w->screen();
+    else if (parentWindow)
+        scrn = parentWindow->screen();
     else if (QGuiApplication::primaryScreen()->virtualSiblings().size() > 1)
         scrn = QGuiApplication::screenAt(QCursor::pos());
     else
@@ -918,6 +926,11 @@ void QDialog::adjustPosition(QWidget* w)
             pp = w->mapToGlobal(QPoint(0,0));
         p = QPoint(pp.x() + w->width()/2,
                     pp.y() + w->height()/ 2);
+    } else if (parentWindow) {
+        // QTBUG-63406: Widget-based dialog in QML, which has no Widget parent
+        // but a transient parent window.
+        QPoint pp = parentWindow->mapToGlobal(QPoint(0, 0));
+        p = QPoint(pp.x() + parentWindow->width() / 2, pp.y() + parentWindow->height() / 2);
     } else {
         // p = middle of the desktop
         p = QPoint(desk.x() + desk.width()/2, desk.y() + desk.height()/2);