QTableView: implement moving rows by drag-n-drop

This is only available when calling setDragDropOverwriteMode(false)
so this makes no difference in most applications (see implementation
of QAbstractItemView::position).

QTableWidget::dropEvent() was assuming we were dropping onto items
without checking that this was the case, this is fixed here,
and QTableView::dropEvent() is added to handle moving rows,
with very similar code to QListView::dropEvent().

Task-number: QTBUG-13873
Task-number: QTBUG-101475
Change-Id: I96e75da2c0abf6f568edba461735905dee5cb84f
Reviewed-by: Richard Moe Gustavsen <richard.gustavsen@qt.io>
(cherry picked from commit 8e63cf282cb248ddd7fbc908766091659b3a0b22)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

src/widgets/itemviews/qtableview.cpp

@@ -3115,6 +3115,68 @@ void QTableView::timerEvent(QTimerEvent *event)
     QAbstractItemView::timerEvent(event);
 }
 
+#if QT_CONFIG(draganddrop)
+/*! \reimp */
+void QTableView::dropEvent(QDropEvent *event)
+{
+    Q_D(QTableView);
+    if (event->source() == this && (event->dropAction() == Qt::MoveAction ||
+                                    dragDropMode() == QAbstractItemView::InternalMove)) {
+        QModelIndex topIndex;
+        int col = -1;
+        int row = -1;
+        // check whether a subclass has already accepted the event, ie. moved the data
+        if (!event->isAccepted() && d->dropOn(event, &row, &col, &topIndex) && !topIndex.isValid() && col != -1) {
+            // Drop between items (reordering) - can only happen with setDragDropOverwriteMode(false)
+            const QModelIndexList indexes = selectedIndexes();
+            QList<QPersistentModelIndex> persIndexes;
+            persIndexes.reserve(indexes.size());
+
+            bool topIndexDropped = false;
+            for (const auto &index : indexes) {
+                // Reorder entire rows
+                QPersistentModelIndex firstColIndex = index.siblingAtColumn(0);
+                if (!persIndexes.contains(firstColIndex))
+                    persIndexes.append(firstColIndex);
+                if (index.row() == topIndex.row()) {
+                    topIndexDropped = true;
+                    break;
+                }
+            }
+            if (!topIndexDropped) {
+                std::sort(persIndexes.begin(), persIndexes.end()); // The dropped items will remain in the same visual order.
+
+                QPersistentModelIndex dropRow = model()->index(row, col, topIndex);
+
+                int r = row == -1 ? model()->rowCount() : (dropRow.row() >= 0 ? dropRow.row() : row);
+                bool dataMoved = false;
+                for (int i = 0; i < persIndexes.size(); ++i) {
+                    const QPersistentModelIndex &pIndex = persIndexes.at(i);
+                    // only generate a move when not same row or behind itself
+                    if (r != pIndex.row() && r != pIndex.row() + 1) {
+                        // try to move (preserves selection)
+                        dataMoved |= model()->moveRow(QModelIndex(), pIndex.row(), QModelIndex(), r);
+                        if (!dataMoved) // can't move - abort and let QAbstractItemView handle this
+                            break;
+                    } else {
+                        // move onto itself is blocked, don't delete anything
+                        dataMoved = true;
+                    }
+                    r = pIndex.row() + 1;   // Dropped items are inserted contiguously and in the right order.
+                }
+                if (dataMoved)
+                    event->accept();
+            }
+        }
+    }
+
+    if (!event->isAccepted()) {
+        // moveRows not implemented, fall back to default
+        QAbstractItemView::dropEvent(event);
+    }
+}
+#endif
+
 /*!
     This slot is called to change the index of the given \a row in the
     table view. The old index is specified by \a oldIndex, and the new

src/widgets/itemviews/qtableview.h

@@ -114,6 +114,10 @@ protected:
 
     void timerEvent(QTimerEvent *event) override;
 
+#if QT_CONFIG(draganddrop)
+    void dropEvent(QDropEvent *event) override;
+#endif
+
     int horizontalOffset() const override;
     int verticalOffset() const override;
     QModelIndex moveCursor(CursorAction cursorAction, Qt::KeyboardModifiers modifiers) override;

src/widgets/itemviews/qtablewidget.cpp

@@ -2722,7 +2722,8 @@ void QTableWidget::dropEvent(QDropEvent *event) {
         int col = -1;
         int row = -1;
         // check whether a subclass has already accepted the event, ie. moved the data
-        if (!event->isAccepted() && d->dropOn(event, &row, &col, &topIndex)) {
+        if (!event->isAccepted() && d->dropOn(event, &row, &col, &topIndex) && row == -1 && col == -1) {
+            // Drop onto item
             const QModelIndexList indexes = selectedIndexes();
             int top = INT_MAX;
             int left = INT_MAX;
@@ -2730,7 +2731,6 @@ void QTableWidget::dropEvent(QDropEvent *event) {
                 top = qMin(index.row(), top);
                 left = qMin(index.column(), left);
             }
-
             QList<QTableWidgetItem *> taken;
             const int indexesCount = indexes.size();
             taken.reserve(indexesCount);
@@ -2745,7 +2745,7 @@ void QTableWidget::dropEvent(QDropEvent *event) {
 
             event->accept();
         }
-        // either we or a subclass accepted the move event, so assume that the data was
+        // either we or a subclass accepted the drop event, so assume that the data was
         // moved and that QAbstractItemView shouldn't remove the source when QDrag::exec returns
         if (event->isAccepted())
             d->dropEventMoved = true;