QDesktopScreenWidget::screenNumber(): Use native geometry.

Coordinates in device independent pixels are not suitable for checking
intersection of screen areas since the screen positions are always
unscaled.

Move the code to determine the QWindow handle out and transform
the frame to device pixels and use QPlatformScreen to check.

Change-Id: Idd94f32abc43bc7a04e4056243b7f810631f70cc
Task-number: QTBUG-52606
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

1 files changed, 12 insertions, 8 deletions

diff --git a/src/widgets/kernel/qdesktopwidget.cpp b/src/widgets/kernel/qdesktopwidget.cpp
index e87b13c630..19e292375a 100644
--- a/src/widgets/kernel/qdesktopwidget.cpp
+++ b/src/widgets/kernel/qdesktopwidget.cpp
@@ -38,6 +38,9 @@
 #include "qwidget_p.h"
 #include "qwindow.h"
 
+#include <private/qhighdpiscaling_p.h>
+#include <qpa/qplatformscreen.h>
+
 QT_BEGIN_NAMESPACE
 
 QDesktopScreenWidget::QDesktopScreenWidget(QScreen *screen, const QRect &geometry)
@@ -239,18 +242,18 @@ int QDesktopWidget::screenNumber(const QWidget *w) const
     if (screens.isEmpty()) // This should never happen
         return primaryScreen();
 
+    const QWindow *winHandle = w->windowHandle();
+    if (!winHandle) {
+        if (const QWidget *nativeParent = w->nativeParentWidget())
+            winHandle = nativeParent->windowHandle();
+    }
+
     // If there is more than one virtual desktop
     if (screens.count() != screens.constFirst()->virtualSiblings().count()) {
         // Find the root widget, get a QScreen from it and use the
         // virtual siblings for checking the window position.
-        const QWidget *root = w;
-        const QWidget *tmp = w;
-        while ((tmp = tmp->parentWidget()))
-            root = tmp;
-        const QWindow *winHandle = root->windowHandle();
         if (winHandle) {
-            const QScreen *winScreen = winHandle->screen();
-            if (winScreen)
+            if (const QScreen *winScreen = winHandle->screen())
                 screens = winScreen->virtualSiblings();
         }
     }
@@ -260,11 +263,12 @@ int QDesktopWidget::screenNumber(const QWidget *w) const
     QRect frame = w->frameGeometry();
     if (!w->isWindow())
         frame.moveTopLeft(w->mapToGlobal(QPoint(0, 0)));
+    const QRect nativeFrame = QHighDpi::toNativePixels(frame, winHandle);
 
     QScreen *widgetScreen = Q_NULLPTR;
     int largestArea = 0;
     foreach (QScreen *screen, screens) {
-        QRect intersected = screen->geometry().intersected(frame);
+        const QRect intersected = screen->handle()->geometry().intersected(nativeFrame);
         int area = intersected.width() * intersected.height();
         if (largestArea < area) {
             widgetScreen = screen;