Unless I am misunderstanding the docs,
tableViewWidget->scrollTo(modelIndex);
QModelIndex modelIndex = tableViewWidget->indexAt(QPoint(0,from));
tableViewWidget->scrollTo(modelIndex);
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Will find the index of the row that is currently at the top-left of the view and scroll to it. This should result in not much movement. I think what you wanted was something like:
QModelIndex modelIndex = model->index(from, 0, QModelIndex());
tableViewWidget->scrollTo(modelIndex, QAbstractItemView::PositionAtTop);
To copy to clipboard, switch view to plain text mode
i.e. find the index of the desired row and scroll to it.
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