QTreeView::setHeaderHidden corrupts column width

[jwu]

A simple rectangular table.
View inherits from QTreeView.
Model inherits from QAbstractTableModel.
There are checkboxes in column one (Model::data responds to role==Qt::CheckStateRole if col==1).
Model::headerData returns the empty default QVariant for col==1.
View::sizeHintForColumn returns fontMetrics().width('m') for col==1.

Everything works like a charm until I activate View::setHeaderHidden(<someCondition>).
Whenever <someCondition> becomes false and the header reappears, column 1 shrinks by about 50%, and the check boxes are clipped.

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Added after 1 11 minutes:

Found the answer: As long as the header is visible, the fontMetrics() reference in View::sizeHintForColumn returns the header font, which is wider than the ordinary font used in the table cells.