Erratic return value QAbstractTextDocumentLayout::documentSize ()

QAbstractTextDocumentLayout::documentSize() will return an an erratic range of sizes even when document is not being changed. This can be demonstrated by running the following example:

By entering some text at about 10 letters or more in the bottom window it will start returning incorrect size values.

///////////////////////////////////////////////////////////////////////////////
// ElasticTextEdit decl //
///////////////////////////////////////////////////////////////////////////////

#include <QtGui/QTextEdit>
#include <QDebug>

/**

• This is a subclass of QTextEdit that changes its preferred height
• as a function of its width and the text it contains. It's otherwise
• the same as QTextEdit.
  */

class ElasticTextEdit : public QTextEdit {
Q_OBJECT

public:
ElasticTextEdit(QWidget* parent = 0);
ElasticTextEdit(const QString& text, QWidget* parent = 0);
~ElasticTextEdit();

// QWidget overrides
int heightForWidth(int width) const;
QSize sizeHint() const;

public slots:
// private
void heightForWidthMayHaveChanged();
} ;

///////////////////////////////////////////////////////////////////////////////
// ElasticTextEdit impl //
///////////////////////////////////////////////////////////////////////////////

#include <QtGui/QAbstractTextDocumentLayout>
#include <QtGui/QSizePolicy>

ElasticTextEdit::ElasticTextEdit(QWidget* parent)
: QTextEdit(parent) {
// Make hasHeightForWidth be enabled by default
QSizePolicy policy(sizePolicy());
policy.setHeightForWidth(true);
setSizePolicy(policy);

connect(this, SIGNAL(textChanged()),
this, SLOT(heightForWidthMayHaveChanged()));
}

ElasticTextEdit::ElasticTextEdit(const QString& text, QWidget* parent)
: QTextEdit(text, parent) {
}

ElasticTextEdit::~ElasticTextEdit() {
}

int
ElasticTextEdit::heightForWidth(int width) const {
QTextDocument* doc = document();
qreal savedWidth = doc->textWidth();

// Fudge factor. This is supposed to be the difference between the
// height of the widget and the height of the area where the text is
// actually drawn. I don't know how to get it exactly.
int fudge = 2;

// Do the calculation assuming no scrollbars
doc->setTextWidth(width);
int noScrollbarHeight =
doc->documentLayout()->documentSize().height() + fudge;
qDebug() << "h: " << doc->documentLayout()->documentSize().height();
qDebug() << "w: " << doc->documentLayout()->documentSize().width();

// (If noScrollbarHeight is greater than the maximum height we'll be
// allowed, then there will be scrollbars, and the actual required
// height will be even higher. But since in this case we've already
// hit the maximum height, it doesn't matter that we underestimate.)

// Get minimum height (even if string is empty): one line of text
int minimumHeight = QFontMetrics(doc->defaultFont()).lineSpacing() + fudge;
int ret = qMax(noScrollbarHeight, minimumHeight);

doc->setTextWidth(savedWidth);
return ret;
}

QSize
ElasticTextEdit::sizeHint() const {
QSize size = QTextEdit::sizeHint();
if (size.width() == 0)
size.setWidth(400); // arbitrary value
size.setHeight(heightForWidth(size.width()));
return size;
}

void
ElasticTextEdit::heightForWidthMayHaveChanged() {
updateGeometry();
}

///////////////////////////////////////////////////////////////////////////////
// test //
///////////////////////////////////////////////////////////////////////////////

#include <QtGui/QApplication>
#include <QtGui/QSizePolicy>
#include <QtGui/QTextEdit>
#include <QtGui/QVBoxLayout>

class Demo : public QWidget {
public:
Demo(QWidget* parent = 0);
} ;

Demo::Demo(QWidget* parent)
: QWidget(parent) {
QVBoxLayout* verticalLayout = new QVBoxLayout(this);

QTextEdit* top = new QTextEdit(this);
QSizePolicy sizePolicy(QSizePolicy::Expanding, QSizePolicy::Expanding);
sizePolicy.setHorizontalStretch(0);
sizePolicy.setVerticalStretch(0);
sizePolicy.setHeightForWidth(top->sizePolicy().hasHeightForWidth());
top->setSizePolicy(sizePolicy);
top->setMinimumSize(QSize(0, 50));
verticalLayout->addWidget(top);

ElasticTextEdit* bottom = new ElasticTextEdit(this);
QSizePolicy sizePolicy1(QSizePolicy::Expanding, QSizePolicy::Preferred);
sizePolicy1.setHorizontalStretch(0);
sizePolicy1.setVerticalStretch(0);
sizePolicy1.setHeightForWidth(bottom->sizePolicy().hasHeightForWidth());
bottom->setSizePolicy(sizePolicy1);
verticalLayout->addWidget(bottom);
}

#include "main.moc"

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

Demo demo;
demo.show();

return app.exec();
}