When you set QHeaderView::defaultSectionSize in a table view such that it looks good on a regular display and then scale your system up in system settings, the cells in the table view are not scaled up while the font size is and now the text doesn't fit to the table cells anymore. See the attached screenshots with normal scale and 300% scale (qtbug_normal.png and qtbug_300.png).

The attached example can be used to reproduce this. Scale the system up (e.g. 300%) to see the problem. Tested only on Windows 10.