Cocoa: fix grabWindow when mixing highDPI and non-highDPI screens

CGDisplayCreateImageForRect seems to have a weird behavior when mixing
highDPI and non-highDPI screens since it may take part of the
non-highDPI screen when the highDPI display is at the left or at the
top of the main monitor.

To workaround this issue, we capture the whole screen and then crop the
image to the desired size.

Task-number: QTBUG-47643
Change-Id: Ib2a3850a0a549964c7fe272abb563bd23518c234
Reviewed-by: Morten Johan Sørvig <[email protected]>

src/plugins/platforms/cocoa/qcocoascreen.mm

@@ -233,25 +233,28 @@ static QString displayName(CGDirectDisplayID displayID)
             windowSize.setHeight(windowRect.height());
     }
 
-    QPixmap windowPixmap(windowSize * devicePixelRatio());
+    const qreal dpr = devicePixelRatio();
+    QPixmap windowPixmap(windowSize * dpr);
     windowPixmap.fill(Qt::transparent);
 
     for (uint i = 0; i < displayCount; ++i) {
         const CGRect bounds = CGDisplayBounds(displays[i]);
-        int w = (width < 0 ? bounds.size.width : width) * devicePixelRatio();
-        int h = (height < 0 ? bounds.size.height : height) * devicePixelRatio();
-        QRect displayRect = QRect(x, y, w, h);
-        displayRect = displayRect.translated(qRound(-bounds.origin.x), qRound(-bounds.origin.y));
-        QCFType<CGImageRef> image = CGDisplayCreateImageForRect(displays[i],
-            CGRectMake(displayRect.x(), displayRect.y(), displayRect.width(), displayRect.height()));
-        QPixmap pix(w, h);
-        pix.fill(Qt::transparent);
-        CGRect rect = CGRectMake(0, 0, w, h);
-        QMacCGContext ctx(&pix);
-        qt_mac_drawCGImage(ctx, &rect, image);
 
+        // Calculate the position and size of the requested area
+        QPoint pos(qAbs(bounds.origin.x - x), qAbs(bounds.origin.y - y));
+        QSize size(qMin(pos.x() + width, qRound(bounds.size.width)),
+                   qMin(pos.y() + height, qRound(bounds.size.height)));
+        pos *= dpr;
+        size *= dpr;
+
+        // Take the whole screen and crop it afterwards, because CGDisplayCreateImageForRect
+        // has a strange behavior when mixing highDPI and non-highDPI displays
+        QCFType<CGImageRef> cgImage = CGDisplayCreateImage(displays[i]);
+        const QImage image = qt_mac_toQImage(cgImage);
+
+        // Draw into windowPixmap only the requested size
         QPainter painter(&windowPixmap);
-        painter.drawPixmap(0, 0, pix);
+        painter.drawImage(windowPixmap.rect(), image, QRect(pos, size));
     }
     return windowPixmap;
 }