It is not possible to interactively decrease section size in horizontal QHeaderView when horizontal scrollbar is in the right-most position.

To reproduce, try the Custom Sort/Filter Model demo. Increase the size of one of the table columns so that horizontal scrollbar appears below the table. Than move the scrollbar to the rightmost position. Then try to decrease the size of any column.

It is actualy possible to decrease the size of the column by a bit, pixel or two in one dragging operation, but that is not enough.

A workaround for the user is to move the scrollbar somewhat, decrease the section somewhat, move the scrollbar again and so on. That is quite confusing and a typical user would not find out.

The technical problem probably is that it is not possible for the section delimiter to actually track the mouse pointer during such dragging operation. The section delimiter has to remain fixed as long as a scrollbar is visible and the user is decreasing since it is not the size of the last column that is being changed and the last column has to stick to the right side of the viewport.

See for example native Mac OS Finder or Windows Explorer for a possible proper behavior.

It is not easy to remedy the problem in a subclass of QHeaderView either because QHeaderView::setOffset, which is an important part of this interaction pattern, is not virtual.