I’m writing a text editor in C++ (Qt Creator 14.0.1). I have managed to add line numbering and line highlighting from examples on internet.

If you start writing from top to bottom line numbering works fine. But if you open a file and put the text into the editor or paste bunch of text into it it will skip numbering multiple lines. here is my code and screenshot:

void CodeEditor::lineNumberAreaPaintEvent(QPaintEvent *event)
{
    QPainter painter(lineNumberArea);
    painter.fillRect(event->rect(), Qt::lightGray);

    QTextBlock block = firstVisibleBlock();
    int blockNumber = block.blockNumber();
    int top = static_cast<int>(blockBoundingGeometry(block).translated(contentOffset()).top());
    int bottom = top + static_cast<int>(blockBoundingRect(block).height());

    // Get the current line number
    int currentLineNumber = textCursor().blockNumber();

    // Loop through each block and draw line numbers
    while (block.isValid() && top <= event->rect().bottom()) {
        if (block.isVisible() && bottom >= event->rect().top()) {
            QString number = QString::number(blockNumber + 1);

            // Check if this is the current line
            if (blockNumber == currentLineNumber) {
                // Set bold font for the current line number
                QFont boldFont = painter.font();
                boldFont.setBold(true);
                painter.setFont(boldFont);
                painter.setPen(Qt::blue); // Optional: change color for current line number
            } else {
                // Use normal font for other line numbers
                painter.setFont(QFont());
                painter.setPen(Qt::black);
            }

            // Adjust the x position to keep numbers left-aligned
            painter.drawText(-5, top, lineNumberArea->width(), fontMetrics().height(),
                             Qt::AlignRight | Qt::AlignVCenter, number);
        }

        block = block.next();
        top = bottom;
        bottom = top + static_cast<int>(blockBoundingRect(block).height());
        ++blockNumber;
    }
}

screenshot

I tried chatgpt and manipulating the algorithm to no avail.