Fix highdpi conversion of QTabletEvent coordinates on xcb

when high-dpi scaling is enabled, there was an offset
from the cursor position to the event position, because
QWindow::mapFromGlobal() works in device-independent
pixels, but we are using actual screen pixels here.

Task-number: QTBUG-77826
Backport from upstream:
    Change-Id: Ic8743b9e5c4041065f530ed1d9d6c49337b0207a
    Commit: 9d51fb579bb4655f6740096f17f1ced50258c28f

src/gui/kernel/qplatformwindow.cpp

@@ -266,6 +266,13 @@ QPoint QPlatformWindow::mapToGlobal(const QPoint &pos) const
     return result;
 }
 
+QPointF QPlatformWindow::mapFromGlobalF(const QPointF &pos) const
+{
+    const QPoint posPt = pos.toPoint();
+    const QPointF delta = pos - posPt;
+    return mapFromGlobal(posPt) + delta;
+}
+
 /*!
     Translates the global screen coordinate \a pos to window
     coordinates using native methods. This is required for embedded windows,

src/gui/kernel/qplatformwindow.h

@@ -114,6 +114,7 @@ public:
     virtual bool isForeignWindow() const { return false; };
     virtual QPoint mapToGlobal(const QPoint &pos) const;
     virtual QPoint mapFromGlobal(const QPoint &pos) const;
+    QPointF mapFromGlobalF(const QPointF &pos) const;
 
     virtual void propagateSizeHints();
 

src/plugins/platforms/xcb/qxcbconnection_xi2.cpp

@@ -1270,16 +1270,16 @@ void QXcbConnection::xi2ReportTabletEvent(const void *event, TabletData *tabletD
             if (Q_LIKELY(useValuators)) {
                 const qreal value = scaleOneValuator(normalizedValue, physicalScreenArea.x(), physicalScreenArea.width());
                 global.setX(value);
-                // mapFromGlobal is ok for nested/embedded windows, but works only with whole-number QPoint;
-                // so map it first, then add back the sub-pixel position
-                local.setX(window->mapFromGlobal(QPoint(int(value), 0)).x() + (value - int(value)));
+                //! \note Fix QTBUG-77826
+                local.setX(xcbWindow->mapFromGlobalF(global).x());
             }
             break;
         case QXcbAtom::AbsY:
             if (Q_LIKELY(useValuators)) {
                 qreal value = scaleOneValuator(normalizedValue, physicalScreenArea.y(), physicalScreenArea.height());
                 global.setY(value);
-                local.setY(window->mapFromGlobal(QPoint(0, int(value))).y() + (value - int(value)));
+                //! \note Fix QTBUG-77826
+                local.setY(xcbWindow->mapFromGlobalF(global).y());
             }
             break;
         case QXcbAtom::AbsPressure: