Why doesn't ColumnView's PreviewWidget fill its container?

[Born Special]

I want the preview widget to automatically fill all the space in the column. I've tried setting layouts on everything. I've tried setting size policy. I can explicitly set the height of the QWidget, but it still doesn't adjust with the size of the window. Here's my code. Please help. Thanks.

#include <QApplication>
#include <QColumnView>
#include <QMainWindow>
#include <QStandardItemModel>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);

    QMainWindow mainwindow;
    QColumnView columns;
    mainwindow.setCentralWidget(&columns);
    mainwindow.setGeometry(100,100,800,800);

    QStandardItemModel model;
    QStandardItem item("Item");
    model.appendRow(&item);
    columns.setModel(&model);

    QWidget previewpane;
    previewpane.setStyleSheet("background-color: red;");
    columns.setPreviewWidget(&previewpane);

    mainwindow.show();
    return a.exec();
}

[SGaist]

Hi and welcome to devnet,

Which version of Qt are you using ?
On which platform ?
How did you install it ?

[Born Special]

Qt 5.15.3 on Linux through Qt Creator.

[SGaist]

Which distribution ?
Which desktop environment ?

[Born Special]

Arch. Openbox.