Invalidating QSortFilterProxyModel filter causes crash when items are selected

I'm trying to make a file browser for a game file system in Qt 5.15.3, with a directory tree view on the left and a file table view on the right. When a directory is selected in the tree, the children of the directory should be displayed in the table, but only the files and not other nested directories.

I have implemented two QSortFilterProxyModels, one for each view. For the tree view, the proxy model's filterAcceptsRow() implementation simply checks whether the model index represents a directory, but the table view's proxy model has to be slightly more complicated since the active root index that the table is using changes when a different directory is selected:

void FileSystemBrowserWidget::onDirectoryActivated(const QModelIndex& index)
{
  // Since the proxy model filters based on the source model, we get
  // the source index to use for filtering. The index we have
  // been passed is from the tree view's proxy model.
  const QModelIndex sourceIndex = m_treeProxyModel->mapToSource(index);

  // Tell the table's proxy model about this new root.
  m_tableProxyModel->setRootForFiltering(sourceIndex);

  // Set the table view's root to display from.
  // The table view shows the proxy model, so we map to this model.
  m_fileSystemTableView->setRootIndex(m_tableProxyModel->mapFromSource(sourceIndex));
}

The table proxy model's setRootForFiltering() function stores this new root, and invalidates the filter. I discovered that I had to do this because the filtering needed to be re-evaluated if the table view's root index was changed. This is because normally a directory would not pass the filter to be shown in the table, and so if it were selected as the root for the table view, this filtering would apply hierarchically and none of the directory's child files would be shown.

void FileSystemBrowserTableProxyModel::setRootForFiltering(const QModelIndex& sourceIndex)
{
  if (m_rootForFiltering == sourceIndex)
  {
    return;
  }

  m_rootForFiltering = sourceIndex;
  invalidateFilter();
}

To fix the filtering issue, the table proxy model's filterAcceptsRow() function allows the item if it matches the root index that's been set:

bool FileSystemBrowserTableProxyModel::filterAcceptsRow(
  int sourceRow,
  const QModelIndex& sourceParent) const
{
  QAbstractItemModel* src = sourceModel();
  const QModelIndex srcIndex = src->index(sourceRow, 0, sourceParent);

  if (srcIndex == m_rootForFiltering)
  {
    // Always allowed, or children don't show up.
    return true;
  }

  // Allow if this index represents a file and not a directory.
  return itemAtIndexIsAFile(srcIndex);
}

This seems to allow filtering to work properly. However, there is a strange issue if I select an item in the table view and then choose a different directory in the tree view. QSortFilterProxyModel: index from wrong model passed to mapFromSource is spammed to my terminal 9 times, and then I get a crash as follows:

___lldb_unnamed_symbol11407 (@___lldb_unnamed_symbol11407:41)
QSortFilterProxyModel::flags(QModelIndex const&) const (@QSortFilterProxyModel::flags(QModelIndex const&) const:43)
QAbstractItemView::focusInEvent(QFocusEvent*) (@QAbstractItemView::focusInEvent(QFocusEvent*):41)
QWidget::event(QEvent*) (@QWidget::event(QEvent*):763)
QFrame::event(QEvent*) (@QFrame::event(QEvent*):14)
QApplicationPrivate::notify_helper(QObject*, QEvent*) (@QApplicationPrivate::notify_helper(QObject*, QEvent*):42)

... call stack continues ...

I've checked my code and the errors about the index coming from the wrong model don't appear to be triggered by calls that I'm making myself. I can only assume that I'm mis-using the proxy models in some way, but I'm not sure exactly how. I'm not adding or removing items from the model, so the underlying data should not be changing.

This issue does not happen if I remove the call to invalidateFilter(). It should also be noted that I can't simply switch on recursiveFilteringEnabled: this means that any visible child will have its parent visible as well, which causes any directories which contain files to pass the filter and show up in the table view. In case it's useful, I'm developing and testing this implementation on Kubuntu.

After some further research, I think I've solved the issue. If a nested directory is being displayed in the table view, all model indices for parent directories in the chain also need to pass the filter, or nothing will display. I'm not quite sure what the exact issue was, but I suspect that the table view was expecting a certain number of indices that it wasn't able to get because I was not catering for the aforementioned rule. Interacting with the view in this state caused some sort of undefined behaviour.

After implementing a check for the above rule, things seem to work again.