HiDPI Drag and Drop: Properly render the default image on Mac

This is only when the attached MIME data contains text, and we
fall back to rendering that text into a pixmap. It requires
getting the device pixel ratio from the source which, for now,
may be a QWidget or a QWindow. Other cases may exist, but that
would bring more dependencies than desired.

Similarly, it fixes the draggabletext example. Other examples
would require either to get updated pixmaps or change substantially
in order to support HiDPI (e.g., the fridgemagnets example).

Change-Id: I66198214233e3e06c87505744e2aaa9691fe1bb6
Reviewed-by: Filipe Azevedo <filipe.azevedo@kdab.com>
Reviewed-by: Timur Pocheptsov <timur.pocheptsov@theqtcompany.com>
Reviewed-by: Morten Johan Sørvig <morten.sorvig@qt.io>

1 files changed, 15 insertions, 1 deletions

diff --git a/src/plugins/platforms/cocoa/qcocoadrag.mm b/src/plugins/platforms/cocoa/qcocoadrag.mm
index 80006ae9b8..1e9c355788 100644
--- a/src/plugins/platforms/cocoa/qcocoadrag.mm
+++ b/src/plugins/platforms/cocoa/qcocoadrag.mm
@@ -34,6 +34,9 @@
 #include "qcocoadrag.h"
 #include "qmacclipboard.h"
 #include "qcocoahelpers.h"
+#ifndef QT_NO_WIDGETS
+#include <QtWidgets/qwidget.h>
+#endif
 
 QT_BEGIN_NAMESPACE
 
@@ -181,7 +184,18 @@ QPixmap QCocoaDrag::dragPixmap(QDrag *drag, QPoint &hotSpot) const
                 const int width = fm.width(s);
                 const int height = fm.height();
                 if (width > 0 && height > 0) {
-                    pm = QPixmap(width, height);
+                    qreal dpr = 1.0;
+                    if (const QWindow *sourceWindow = qobject_cast<QWindow *>(drag->source())) {
+                        dpr = sourceWindow->devicePixelRatio();
+                    }
+#ifndef QT_NO_WIDGETS
+                    else if (const QWidget *sourceWidget = qobject_cast<QWidget *>(drag->source())) {
+                        if (const QWindow *sourceWindow = sourceWidget->window()->windowHandle())
+                            dpr = sourceWindow->devicePixelRatio();
+                    }
+#endif
+                    pm = QPixmap(width * dpr, height * dpr);
+                    pm.setDevicePixelRatio(dpr);
                     QPainter p(&pm);
                     p.fillRect(0, 0, pm.width(), pm.height(), Qt::color0);
                     p.setPen(Qt::color1);