setAlternatingRowColors(true) and header()->setStretchLastSection(false) in QTreeView draws empty lines up to the right border of the viewport and full lines only up to the last column.

Try for example the treeview demo with header()->setStretchLastSection(false). Then expand the windows so that there is an empty space behind the last column and below the last line. You will see that the background for every other line draws differently in the full lines and the empty lines.