DragHandler: keep the grab (prevent stealing) when dragging

Whenever the DragHandler gets an exclusive grab, it needs to prevent
a parent Flickable (for example) from taking over that grab.  Since
it waits until the drag threshold is exceeded in an allowed dragging
direction, this does not prevent the Flickable from being draggable
in the other direction.

We restore the state of keepTouchGrab and keepMouseGrab when ungrabbing.

Change-Id: Id9d456c99322e0cb6996d1f690b38fcd6becc6f9
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>

3 files changed, 20 insertions, 0 deletions

diff --git a/src/quick/handlers/qquickdraghandler.cpp b/src/quick/handlers/qquickdraghandler.cpp
index 9575f7ede9..db17df36ca 100644
--- a/src/quick/handlers/qquickdraghandler.cpp
+++ b/src/quick/handlers/qquickdraghandler.cpp
@@ -98,6 +98,14 @@ void QQuickDragHandler::handleEventPoint(QQuickEventPoint *point)
         } else if ((m_xAxis.enabled() && QQuickWindowPrivate::dragOverThreshold(delta.x(), Qt::XAxis, point)) ||
                    (m_yAxis.enabled() && QQuickWindowPrivate::dragOverThreshold(delta.y(), Qt::YAxis, point))) {
             setExclusiveGrab(point);
+            if (target()) {
+                if (point->pointerEvent()->asPointerTouchEvent())
+                    target()->setKeepTouchGrab(true);
+                // tablet and mouse are treated the same by Item's legacy event handling, and
+                // touch becomes synth-mouse for Flickable, so we need to prevent stealing
+                // mouse grab too, whenever dragging occurs in an enabled direction
+                target()->setKeepMouseGrab(true);
+            }
         }
     } break;
     default:
diff --git a/src/quick/handlers/qquickpointerhandler.cpp b/src/quick/handlers/qquickpointerhandler.cpp
index 641324e61e..90d07b5d6d 100644
--- a/src/quick/handlers/qquickpointerhandler.cpp
+++ b/src/quick/handlers/qquickpointerhandler.cpp
@@ -62,6 +62,8 @@ QQuickPointerHandler::QQuickPointerHandler(QObject *parent)
   , m_enabled(true)
   , m_active(false)
   , m_targetExplicitlySet(false)
+  , m_hadKeepMouseGrab(false)
+  , m_hadKeepTouchGrab(false)
 {
 }
 
@@ -92,12 +94,20 @@ void QQuickPointerHandler::setExclusiveGrab(QQuickEventPoint *point, bool grab)
 {
     QQuickPointerHandler *oldGrabber = point->grabberPointerHandler();
     if (grab && oldGrabber != this) {
+        if (target()) {
+            m_hadKeepMouseGrab = target()->keepMouseGrab();
+            m_hadKeepTouchGrab = target()->keepTouchGrab();
+        }
         if (oldGrabber)
             oldGrabber->handleGrabCancel(point);
         point->setGrabberPointerHandler(this, true);
         onGrabChanged(point);
 //        emit grabChanged(point); // TODO maybe
     } else if (!grab && oldGrabber == this) {
+        if (auto tgt = target()) {
+            tgt->setKeepMouseGrab(m_hadKeepMouseGrab);
+            tgt->setKeepTouchGrab(m_hadKeepTouchGrab);
+        }
         point->setGrabberPointerHandler(nullptr, true);
         onGrabChanged(point);
 //        emit grabChanged(point); // TODO maybe
diff --git a/src/quick/handlers/qquickpointerhandler_p.h b/src/quick/handlers/qquickpointerhandler_p.h
index 0931cfcfad..39362ca2e0 100644
--- a/src/quick/handlers/qquickpointerhandler_p.h
+++ b/src/quick/handlers/qquickpointerhandler_p.h
@@ -113,6 +113,8 @@ private:
     bool m_enabled : 1;
     bool m_active : 1;
     bool m_targetExplicitlySet : 1;
+    bool m_hadKeepMouseGrab : 1;    // some handlers override target()->setKeepMouseGrab(); this remembers previous state
+    bool m_hadKeepTouchGrab : 1;    // some handlers override target()->setKeepTouchGrab(); this remembers previous state
 
     friend class QQuickEventPoint;
 };