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, 4 insertions, 0 deletions

diff --git a/src/plugins/platforms/windows/qwindowsdrag.cpp b/src/plugins/platforms/windows/qwindowsdrag.cpp
index b7d225cb00..ee82b2f022 100644
--- a/src/plugins/platforms/windows/qwindowsdrag.cpp
+++ b/src/plugins/platforms/windows/qwindowsdrag.cpp
@@ -652,6 +652,7 @@ QWindowsOleDropTarget::Drop(LPDATAOBJECT pDataObj, DWORD grfKeyState,
 */
 
 bool QWindowsDrag::m_canceled = false;
+bool QWindowsDrag::m_dragging = false;
 
 QWindowsDrag::QWindowsDrag() = default;
 
@@ -699,7 +700,10 @@ Qt::DropAction QWindowsDrag::drag(QDrag *drag)
     const DWORD allowedEffects = translateToWinDragEffects(possibleActions);
     qCDebug(lcQpaMime) << '>' << __FUNCTION__ << "possible Actions=0x"
         << hex << int(possibleActions) << "effects=0x" << allowedEffects << dec;
+    // Indicate message handlers we are in DoDragDrop() event loop.
+    QWindowsDrag::m_dragging = true;
     const HRESULT r = DoDragDrop(dropDataObject, windowDropSource, allowedEffects, &resultEffect);
+    QWindowsDrag::m_dragging = false;
     const DWORD  reportedPerformedEffect = dropDataObject->reportedPerformedEffect();
     if (r == DRAGDROP_S_DROP) {
         if (reportedPerformedEffect == DROPEFFECT_MOVE && resultEffect != DROPEFFECT_MOVE) {