The first block in a QTextEdit object does not display the top margin.

#include "MainWindow.h"
#include "ui_mainwindow.h"
#include <QHBoxLayout>
#include <QTextEdit>

MainWindow::MainWindow(QWidget *parent)
 : QMainWindow(parent),
   ui(new Ui::MainWindow),
   m_TextEdit(new QTextEdit(this))
{
   ui->setupUi(this);

   QHBoxLayout *layout = new QHBoxLayout(ui->centralwidget);
   layout->addWidget(m_TextEdit, 1, Qt::AlignHCenter);

   QTextCursor cursor(m_TextEdit->document());

   QTextBlockFormat blockFormat;
   blockFormat.setTopMargin(30.0);
   blockFormat.setBottomMargin(30.0);

   cursor.insertText("First block of text."); // Write in first (default) block.

   cursor.insertBlock();
   cursor.insertText("Second block of text.");

   cursor.insertBlock();
   cursor.insertText("Third block of text.");

   cursor.select(QTextCursor::Document);
   cursor.setBlockFormat(blockFormat);

   // => First block lacks the top margin.
}

MainWindow::~MainWindow()
{
 delete ui;
 delete m_TextEdit;
}