[KevinH]

Not a bug. It works as designed. The code, just like other filters used in Sigil, is purposely made case insensitive via a lower casing.

void SpellcheckEditor::FilterEditTextChangedSlot(const QString &text)
{
    const QString lowercaseText = text.toLower();
    QModelIndex root_index = m_SpellcheckEditorModel->indexFromItem(m_SpellcheckEditorModel->invisibleRootItem());

    for (int row = 0; row < m_SpellcheckEditorModel->invisibleRootItem()->rowCount(); row++) {
        QStandardItem *item = m_SpellcheckEditorModel->item(row, 0);
        bool hidden = !(text.isEmpty() || item->text().toLower().contains(lowercaseText));
        ui.SpellcheckEditorTree->setRowHidden(item->row(), root_index, hidden);
    }
}