Windows QPA: Fix Drag&Drop from touchscreen/pen

The Drag&Drop functionality had stopped working with touchscreen/pen
after the WM_POINTER-based input handling was added. The Drag&Drop
functionality internally uses the DoDragDrop() WIN32 call which,
according to Microsoft docs, is not supported for invocation inside
handlers for touch/pen messages, and should be invoked in handlers
for mouse messages that are synthesized by the OS afterwards. The
result was that when DoDragDrop (which is a blocking function with
its own event loop) was called it would hang ignoring all touch/pen
messages until a mouse/touchpad message arrived. This change
implements a workaround for this issue by enqueuing Qt touch/pen
events that would be generated inside the pointer message handler,
and that could start a Drag&Drop operation, and only producing them
after the OS sends the associated mouse messages.

Task-number: QTBUG-70887
Change-Id: Id45e0ecc70358ba250de9b3268856781ed21c9dd
Reviewed-by: Friedemann Kleint <Friedemann.Kleint@qt.io>

1 files changed, 2 insertions, 0 deletions

diff --git a/src/plugins/platforms/windows/qwindowsdrag.h b/src/plugins/platforms/windows/qwindowsdrag.h
index f116e50cbf..5f30c59882 100644
--- a/src/plugins/platforms/windows/qwindowsdrag.h
+++ b/src/plugins/platforms/windows/qwindowsdrag.h
@@ -92,6 +92,7 @@ public:
     static QWindowsDrag *instance();
     void cancelDrag() override { QWindowsDrag::m_canceled = true; }
     static bool isCanceled() { return QWindowsDrag::m_canceled; }
+    static bool isDragging() { return QWindowsDrag::m_dragging; }
 
     IDataObject *dropDataObject() const             { return m_dropDataObject; }
     void setDropDataObject(IDataObject *dataObject) { m_dropDataObject = dataObject; }
@@ -102,6 +103,7 @@ public:
 
 private:
     static bool m_canceled;
+    static bool m_dragging;
 
     QWindowsDropMimeData m_dropData;
     IDataObject *m_dropDataObject = nullptr;