TableView does not take header delegate implicitWidth into account to calculate column width, so it may become cut. So Row heights and column widths calculations only related to the TableView columns/delegates, and not  to the connected HorizontalHeaderView, which is wrong.

To reproduce  you may take this example: https://github.com/akontsevich/learnqt_blog_posts/tree/fix_errors/01-TableViewCppModel

And set TableView.delegate Label text property to something short:

text: "1"

All the headers are cut:

In Label item implicitWidth property is readonly. Issue is also reproducible with different controls with implicitWidth explicitly set.