QAbstractItemView: don't lose items if model only allows MoveAction

If a model only allows MoveAction, then calls in the view/widget subclasses'
dropEvent implementation to set the event's drop action to CopyAction
will fail. QAbstractItemView will then remove the item when QDrag::exec
returns.

Instead of abusing the event actions for this, store explicitly that the
dropEvent implementation already moved the item. If the flag is set,
don't remove the item.

In QListView, which uses moveRow to move items in the dropEvent handler,
handle the case that the model might not implement moveRows. In that
case, or when dropping an item onto another item (to overwrite data),
fall back to the default implementation of QAbstractItemView. Sadly, it
is impossible to know whether a model doesn't implement moveRows, or
whether the move failed for other reasons, so this requires a bit of
extra special case handling. QListView in IconMode is particularly odd
in that it moves the item in the view, but not in the model.

This follows up on fd894fd68edf3d67975cda8eb9dda43646887b0d and fixes
additional issues discovered during debugging. Extend the existing unit
test; since drag'n'drop runs a modal, native event loop on most systems,
it still only runs on the Xcb platform.

Change-Id: I6c5377e2b097c8080001afe904d6d3e4aed33df4
Fixes: QTBUG-87057
Reviewed-by: Jan Arve Sæther <jan-arve.saether@qt.io>
(cherry picked from commit 0f1008a5936c903ca9448193df7df6117e2c617b)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

1 files changed, 4 insertions, 1 deletions

diff --git a/src/widgets/itemviews/qabstractitemview.cpp b/src/widgets/itemviews/qabstractitemview.cpp
index 3a11ff4a90..b8cc5621fb 100644
--- a/src/widgets/itemviews/qabstractitemview.cpp
+++ b/src/widgets/itemviews/qabstractitemview.cpp
@@ -98,6 +98,7 @@ QAbstractItemViewPrivate::QAbstractItemViewPrivate()
         dragEnabled(false),
         dragDropMode(QAbstractItemView::NoDragDrop),
         overwrite(false),
+        dropEventMoved(false),
         dropIndicatorPosition(QAbstractItemView::OnItem),
         defaultDropAction(Qt::IgnoreAction),
 #endif
@@ -3711,8 +3712,10 @@ void QAbstractItemView::startDrag(Qt::DropActions supportedActions)
             defaultDropAction = d->defaultDropAction;
         else if (supportedActions & Qt::CopyAction && dragDropMode() != QAbstractItemView::InternalMove)
             defaultDropAction = Qt::CopyAction;
-        if (drag->exec(supportedActions, defaultDropAction) == Qt::MoveAction)
+        d->dropEventMoved = false;
+        if (drag->exec(supportedActions, defaultDropAction) == Qt::MoveAction && !d->dropEventMoved)
             d->clearOrRemove();
+        d->dropEventMoved = false;
         // Reset the drop indicator
         d->dropIndicatorRect = QRect();
         d->dropIndicatorPosition = OnItem;