QHeaderView sectionResize does not work correctly

In Qt 5.12.2 it is not possible to resize a certain section below a certain minimum size. Up to Qt 5.9.2 this worked perfectly. The following pirctures show this bug. The first column only displays icons via the decoration role. The first column is resized to a fixed size via the following code:

static const int Padding = 3;
Header->setSectionResizeMode(0, QHeaderView::Fixed);
Header->resizeSection(0, ui->tableView->iconSize().width() + 2 * Padding);

This is the result for Qt 5.9.2

This is the result for Qt 5.12.2

The view always reserves a minimum size for the first column.

The attached test case shows this problem.