QTreeView Horizontal Scrollbar issue

[Dehumanizer]

I've a problem with QTreeView horizontal scrollbar, it doesn't appear. I've set horizontal scrollbar policy to ScrollBarAsNeeded, but it doesn't appear if needed. Have tried to connect expanded and collapsed signals to a slot:

connect(this, SIGNAL(expanded(QModelIndex)), this, SLOT(update_scroll_area(QModelIndex)));
connect(this, SIGNAL(collapsed(QModelIndex)), this, SLOT(update_scroll_area(QModelIndex)));

connect(this, SIGNAL(expanded(QModelIndex)), this, SLOT(update_scroll_area(QModelIndex)));
connect(this, SIGNAL(collapsed(QModelIndex)), this, SLOT(update_scroll_area(QModelIndex)));

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The slot consists of one line of code:

void update_scroll_area(const QModelIndex& i)
{
        resizeColumnToContents(i.column());
}

void update_scroll_area(const QModelIndex& i)
{
        resizeColumnToContents(i.column());
}

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This makes scrollbar working, but only when I'm expanding/collapsing the tree view items.

I need to have working horizontal scrollbar "every time", from starting the application till its end. How can it be organized?

Thank you.

UPD: Have tried this code, but didn't help:

header()->setHorizontalScrollMode(QAbstractItemView::ScrollPerPixel);
header()->setResizeMode(QHeaderView::ResizeToContents);
header()->setStretchLastSection(false);

header()->setHorizontalScrollMode(QAbstractItemView::ScrollPerPixel);
header()->setResizeMode(QHeaderView::ResizeToContents);
header()->setStretchLastSection(false);

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