Distribute QHeaderView section sizes more evenly in Stretch resize mode

Currently, when using QHeaderView::Stretch as resize mode, it adjusts section sizes based on simple division, with a pixelRemainder that is continuously reduced until it's zero.

The result is that we may end up with headers that have the sections sizes formally divided into two groups: the topmost (or leftmost) having a section size of x (normally greater than 1), and the remaining having having a size of x-1.

While normally not a problem (for normally sized headers the visual difference is not noticeable), it can become a issue in some cases, visually but not only:

• multiple selection by dragging in a table view will cause very different amounts of selected items for a similar amount of movement;
• the selection of the same amount of items is not visually similar depending on their position;
• a QHeaderView that may display groups of sections (for instance, a different background every 20 sections) will cause very different group sizes;

I propose to use a slightly different approach for this, using a partially incremental-error based algorithm. It may not be perfect, but it's more evenly and consistently distributed along the header length.

I applied the following approach in a custom subclass, which is called whenever necessary, including within resizeEvent of the view (forgive me, but I'm not able to write proper C++ code, so I'll simply show the basic idea as a form of pseudo-code written in Python):

def resizeStretch(self):
    header = self.verticalHeader()
    updates = header.updatesEnabled()
    if updates:
        header.setUpdatesEnabled(False)

    availableSize = self.viewport().height()
    count = self.model().rowCount()
    step: float = availableSize / count
    old = 0
    for i in range(1, count + 1):
        new = round(i * step)
        header.resizeSection(i, new - old)
        old = new

    if updates:
        header.setUpdatesEnabled(True)

The two attached images show a table with 100 rows and a size causing the viewport having a height of 350 pixels. In the first case, the default Stretch behavior is shown, in the second the above algorithm is applied using the Fixed mode. I selected 20 rows both on top and bottom.