Wrong checkbox geometry when used in custom delegate in QTreeView

There is a QTreeView with a custom delegate. The delegate has two items: checkbox and label. When checkbox is not checked everything works fine.

When user clicks on the checkbox its gets enlarged and goes beyond boundary of the delegate. This behavior is only visible if checkbox is used as a part of a delegate, normally checkbox has the same size no matter whether it is checked or not.

In the attached example a QTreeView with one item is created. Click on the checkbox and you will see that it gets enlarged.