Windows QPA: Remove tablet->mouse synth and fix Drag&Drop with touch/pen

This patch avoids synthesizing mouse messages in the QPA for touch/pen
input, and lets the GUI do mouse event synthesis for unhandled touch/pen
events, if required, like in other platforms. This requires a workaround
to avoid breaking drag and drop with touch/pen (or making it worse). DnD
on Windows is based on the DoDragDrop() Win32 API, which does not work
with touch/pen input, which in some cases cause a DnD operation started
with touch/pen to hang until the mouse is moved. To avoid it we process
pointer messages for touch/pen and generate mouse input through
SendInput() to trigger DoDragDrop(), which then seems to work as
expected.

So now we inform QtGui that the Windows platform no longer sends
synth-mouse events after tablet events (unsetting the flag added in
f931e5e72d4617023bbea46cba2c0d61bb1efa4f); this completes what was
attempted in 8ada0633cd58e0608df2e16880f1293286025504.

Fixes: QTBUG-106368
Fixes: QTBUG-57577
Fixes: QTBUG-100788
Task-number: QTBUG-77414
Task-number: QTBUG-104594
Pick-to: 6.4 6.3 6.2
Change-Id: I46db3c74be2a95cf2d94ba930398e58dc930d2db
Reviewed-by: Shawn Rutledge <shawn.rutledge@qt.io>

3 files changed, 82 insertions, 3 deletions

diff --git a/src/plugins/platforms/windows/qwindowsdrag.cpp b/src/plugins/platforms/windows/qwindowsdrag.cpp
index 33b1cf2abd..93205259f8 100644
--- a/src/plugins/platforms/windows/qwindowsdrag.cpp
+++ b/src/plugins/platforms/windows/qwindowsdrag.cpp
@@ -644,6 +644,84 @@ IDropTargetHelper* QWindowsDrag::dropHelper() {
     return m_cachedDropTargetHelper;
 }
 
+// Workaround for DoDragDrop() not working with touch/pen input, causing DnD to hang until the mouse is moved.
+// We process pointer messages for touch/pen and generate mouse input through SendInput() to trigger DoDragDrop()
+static HRESULT startDoDragDrop(LPDATAOBJECT pDataObj, LPDROPSOURCE pDropSource, DWORD dwOKEffects, LPDWORD pdwEffect)
+{
+    HWND hwnd = ::GetFocus();
+    bool starting = false;
+
+    for (;;) {
+        MSG msg{};
+        if (::GetMessage(&msg, hwnd, 0, 0) > 0) {
+
+            if (msg.message == WM_MOUSEMOVE) {
+
+                // Only consider the first simulated event, or actual mouse messages.
+                if (!starting && (msg.wParam & (MK_LBUTTON | MK_MBUTTON | MK_RBUTTON | MK_XBUTTON1 | MK_XBUTTON2)) == 0)
+                    return E_FAIL;
+
+                return ::DoDragDrop(pDataObj, pDropSource, dwOKEffects, pdwEffect);
+            }
+
+            if (msg.message == WM_POINTERUPDATE) {
+
+                const quint32 pointerId = GET_POINTERID_WPARAM(msg.wParam);
+
+                POINTER_INFO pointerInfo{};
+                if (!GetPointerInfo(pointerId, &pointerInfo))
+                    return E_FAIL;
+
+                if (pointerInfo.pointerFlags & POINTER_FLAG_PRIMARY) {
+
+                    DWORD flags = MOUSEEVENTF_ABSOLUTE | MOUSEEVENTF_VIRTUALDESK | MOUSEEVENTF_MOVE;
+                    if (IS_POINTER_FIRSTBUTTON_WPARAM(msg.wParam))
+                        flags |= MOUSEEVENTF_LEFTDOWN;
+                    if (IS_POINTER_SECONDBUTTON_WPARAM(msg.wParam))
+                        flags |= MOUSEEVENTF_RIGHTDOWN;
+                    if (IS_POINTER_THIRDBUTTON_WPARAM(msg.wParam))
+                        flags |= MOUSEEVENTF_MIDDLEDOWN;
+
+                    if (!starting) {
+                        POINT pt{};
+                        if (::GetCursorPos(&pt)) {
+
+                            // Send mouse input that can generate a WM_MOUSEMOVE message.
+                            if ((flags & MOUSEEVENTF_LEFTDOWN || flags & MOUSEEVENTF_RIGHTDOWN || flags & MOUSEEVENTF_MIDDLEDOWN)
+                                && (pt.x != pointerInfo.ptPixelLocation.x || pt.y != pointerInfo.ptPixelLocation.y)) {
+
+                                const int origin_x = ::GetSystemMetrics(SM_XVIRTUALSCREEN);
+                                const int origin_y = ::GetSystemMetrics(SM_YVIRTUALSCREEN);
+                                const int virt_w = ::GetSystemMetrics(SM_CXVIRTUALSCREEN);
+                                const int virt_h = ::GetSystemMetrics(SM_CYVIRTUALSCREEN);
+                                const int virt_x = pointerInfo.ptPixelLocation.x - origin_x;
+                                const int virt_y = pointerInfo.ptPixelLocation.y - origin_y;
+
+                                INPUT input{};
+                                input.type = INPUT_MOUSE;
+                                input.mi.dx = static_cast<DWORD>(virt_x * (65535.0 / virt_w));
+                                input.mi.dy = static_cast<DWORD>(virt_y * (65535.0 / virt_h));
+                                input.mi.dwFlags = flags;
+
+                                ::SendInput(1, &input, sizeof(input));
+                                starting = true;
+                            }
+                        }
+                    }
+                }
+            } else {
+                // Handle other messages.
+                qWindowsWndProc(msg.hwnd, msg.message, msg.wParam, msg.lParam);
+
+                if (msg.message == WM_POINTERLEAVE)
+                    return E_FAIL;
+            }
+        } else {
+            return E_FAIL;
+        }
+    }
+}
+
 Qt::DropAction QWindowsDrag::drag(QDrag *drag)
 {
     // TODO: Accessibility handling?
@@ -661,7 +739,7 @@ Qt::DropAction QWindowsDrag::drag(QDrag *drag)
         << Qt::hex << int(possibleActions) << "effects=0x" << allowedEffects << Qt::dec;
     // Indicate message handlers we are in DoDragDrop() event loop.
     QWindowsDrag::m_dragging = true;
-    const HRESULT r = DoDragDrop(dropDataObject, windowDropSource, allowedEffects, &resultEffect);
+    const HRESULT r = startDoDragDrop(dropDataObject, windowDropSource, allowedEffects, &resultEffect);
     QWindowsDrag::m_dragging = false;
     const DWORD  reportedPerformedEffect = dropDataObject->reportedPerformedEffect();
     if (r == DRAGDROP_S_DROP) {
diff --git a/src/plugins/platforms/windows/qwindowsintegration.cpp b/src/plugins/platforms/windows/qwindowsintegration.cpp
index 2faeb7456b..dfbf5609fb 100644
--- a/src/plugins/platforms/windows/qwindowsintegration.cpp
+++ b/src/plugins/platforms/windows/qwindowsintegration.cpp
@@ -223,6 +223,7 @@ void QWindowsIntegrationPrivate::parseOptions(QWindowsIntegration *q, const QStr
         QCoreApplication::setAttribute(Qt::AA_CompressHighFrequencyEvents);
     else
         m_context.initTablet();
+    QWindowSystemInterfacePrivate::TabletEvent::setPlatformSynthesizesMouse(false);
 
     if (!dpiAwarenessSet) { // Set only once in case of repeated instantiations of QGuiApplication.
         if (!QCoreApplication::testAttribute(Qt::AA_PluginApplication)) {
diff --git a/src/plugins/platforms/windows/qwindowspointerhandler.cpp b/src/plugins/platforms/windows/qwindowspointerhandler.cpp
index 89f236b8ee..88f02347b3 100644
--- a/src/plugins/platforms/windows/qwindowspointerhandler.cpp
+++ b/src/plugins/platforms/windows/qwindowspointerhandler.cpp
@@ -796,8 +796,8 @@ bool QWindowsPointerHandler::translateMouseEvent(QWindow *window,
             break;
         case QT_PT_PEN:
 #if QT_CONFIG(tabletevent)
-            if (!m_activeTabletDevice.isNull())
-                device = m_activeTabletDevice.data();
+            qCDebug(lcQpaTablet) << "ignoring synth-mouse event for tablet event from" << device;
+            return false;
 #endif
             break;
         }