Windows QPA: Fix scaling of tablet coordinates for High DPI scaling

Determine the virtual desktop geometry in native pixels from
the QPlatformScreen siblings. Use the platform window to determine
the local position.

Task-number: QTBUG-36937
Change-Id: Ic250f1b41d8e74099558796112dbd70a7e1f6956
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

3 files changed, 16 insertions, 2 deletions

diff --git a/src/plugins/platforms/windows/qwindowsscreen.cpp b/src/plugins/platforms/windows/qwindowsscreen.cpp
index c0781df973..66d3b140e2 100644
--- a/src/plugins/platforms/windows/qwindowsscreen.cpp
+++ b/src/plugins/platforms/windows/qwindowsscreen.cpp
@@ -318,6 +318,15 @@ void QWindowsScreen::handleChanges(const QWindowsScreenData &newData)
     }
 }
 
+QRect QWindowsScreen::virtualGeometry(const QPlatformScreen *screen) // cf QScreen::virtualGeometry()
+{
+    QRect result;
+    const auto siblings = screen->virtualSiblings();
+    for (const QPlatformScreen *sibling : siblings)
+        result |= sibling->geometry();
+    return result;
+}
+
 enum OrientationPreference // matching Win32 API ORIENTATION_PREFERENCE
 #if defined(Q_COMPILER_CLASS_ENUM) || defined(Q_CC_MSVC)
     : DWORD
diff --git a/src/plugins/platforms/windows/qwindowsscreen.h b/src/plugins/platforms/windows/qwindowsscreen.h
index 7cf73f03af..5753e605da 100644
--- a/src/plugins/platforms/windows/qwindowsscreen.h
+++ b/src/plugins/platforms/windows/qwindowsscreen.h
@@ -113,6 +113,8 @@ public:
 
     const QWindowsScreenData &data() const  { return m_data; }
 
+    static QRect virtualGeometry(const QPlatformScreen *screen);
+
 private:
     QWindowsScreenData m_data;
 #ifndef QT_NO_CURSOR
diff --git a/src/plugins/platforms/windows/qwindowstabletsupport.cpp b/src/plugins/platforms/windows/qwindowstabletsupport.cpp
index 5fdc664603..78e33e57ab 100644
--- a/src/plugins/platforms/windows/qwindowstabletsupport.cpp
+++ b/src/plugins/platforms/windows/qwindowstabletsupport.cpp
@@ -421,7 +421,8 @@ bool QWindowsTabletSupport::translateTabletPacketEvent()
     //    in which case we snap the position to the mouse position.
     // It seems there is no way to find out the mode programmatically, the LOGCONTEXT orgX/Y/Ext
     // area is always the virtual desktop.
-    const QRect virtualDesktopArea = QGuiApplication::primaryScreen()->virtualGeometry();
+    const QRect virtualDesktopArea =
+        QWindowsScreen::virtualGeometry(QGuiApplication::primaryScreen()->handle());
 
     qCDebug(lcQpaTablet) << __FUNCTION__ << "processing " << packetCount
         << "target:" << QGuiApplicationPrivate::tabletDevicePoint(uniqueId).target;
@@ -455,7 +456,9 @@ bool QWindowsTabletSupport::translateTabletPacketEvent()
         if (!target)
             continue;
 
-        const QPoint localPos = target->mapFromGlobal(globalPos);
+        const QPlatformWindow *platformWindow = target->handle();
+        Q_ASSERT(platformWindow);
+        const QPoint localPos = platformWindow->mapFromGlobal(globalPos);
 
         const qreal pressureNew = packet.pkButtons && (currentPointer == QTabletEvent::Pen || currentPointer == QTabletEvent::Eraser) ?
             m_devices.at(m_currentDevice).scalePressure(packet.pkNormalPressure) :