QPainter draw semi-transparent overlay watermark changes lines underneath in PDF

[zhenfred.chen]

I'm trying to draw a watermark on top of some existing lines and texts. The watermark is drawn semi-transparently so the watermark text should blend with lines underneath. It looks fine in Print Preview. But when I send it to the Microsoft Print to PDF printer, the resulting PDF shows thinned lines underneath the watermark. I expected the lines that do not intersect with the watermark text to be unchanged.

Qt Creator 11
Qt 6.5.1 mingw64
Windows 10

PrinterTest.pro:

QT       += core gui printsupport

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++17

# You can make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

SOURCES += \
    main.cpp \
    mainwindow.cpp

HEADERS += \
    mainwindow.h

FORMS += \
    mainwindow.ui

# Default rules for deployment.
qnx: target.path = /tmp/$${TARGET}/bin
else: unix:!android: target.path = /opt/$${TARGET}/bin
!isEmpty(target.path): INSTALLS += target

mainwindow.h:

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>
#include <QPrinter>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private slots:
    void onBtnPrintClicked();
    void onBtnPrintPreviewClicked();

private:
    Ui::MainWindow *ui;

    void print(QPrinter *printer);
};
#endif // MAINWINDOW_H

mainwindow.cpp:

#include "mainwindow.h"
#include "ui_mainwindow.h"

#include <QPrintDialog>
#include <QPrintPreviewDialog>
#include <QPainter>
#include <QDebug>

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

    connect(ui->btnPrint, &QPushButton::clicked, this, &MainWindow::onBtnPrintClicked);
    connect(ui->btnPrintPreview, &QPushButton::clicked, this, &MainWindow::onBtnPrintPreviewClicked);
}

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

void MainWindow::onBtnPrintClicked()
{
    QPrinter printer(QPrinter::HighResolution);
    QPrintDialog dialog(&printer, this);
    dialog.setFromTo(1, 2);
    if (dialog.exec() != QDialog::Accepted) return;
    print(&printer);
}

void MainWindow::onBtnPrintPreviewClicked()
{
    QPrinter printer(QPrinter::HighResolution);
    printer.setFromTo(1, 2);
    QPrintPreviewDialog preview(&printer, this);
    preview.setWindowFlags(preview.windowFlags() & ~Qt::WindowContextHelpButtonHint);
    connect(&preview, &QPrintPreviewDialog::paintRequested, this, &MainWindow::print);
    preview.exec();
}

void MainWindow::print(QPrinter *printer)
{
    QPainter painter;
    if (!painter.begin(printer)) {
        return;
    }

    QRectF pageRect = printer->pageRect(QPrinter::DevicePixel);
    double pageMargin = 0.04 * pageRect.width();
    QRect contentRect = QRect(0, 0,
                              pageRect.width() - 2 * pageMargin,
                              pageRect.height() - 2 * pageMargin);

    painter.translate(pageMargin, pageMargin);
    painter.setPen(QPen(Qt::black, 1));

    // Draw some horizontal lines
    int y = contentRect.height() / 3;
    while (y < contentRect.height() - contentRect.height() / 3) {
        painter.drawLine(0, y, contentRect.width(), y);
        y += 100;
    }

    // Draw a watermark that's on top of the lines
    QFont font;
    font.setPointSize(50);
    font.setWeight(QFont::Bold);
    float a = -qRadiansToDegrees(atan2(pageRect.height(), pageRect.width()));
    QString text = "W A T E R M A R K";
    painter.setBrush(Qt::transparent);
    painter.setFont(font);
    painter.translate(pageRect.width()/2, pageRect.height()/2);
    painter.rotate(a);
    painter.setPen(QColor(175, 175, 175, 75));
    painter.drawText(-painter.fontMetrics().horizontalAdvance(text)/2, painter.fontMetrics().height()/2, text);

    painter.end();
}

mainwindow.ui:

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>474</width>
    <height>478</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralwidget">
   <widget class="QPushButton" name="btnPrintPreview">
    <property name="geometry">
     <rect>
      <x>150</x>
      <y>140</y>
      <width>181</width>
      <height>71</height>
     </rect>
    </property>
    <property name="text">
     <string>Print Preview</string>
    </property>
   </widget>
   <widget class="QPushButton" name="btnPrint">
    <property name="geometry">
     <rect>
      <x>150</x>
      <y>230</y>
      <width>181</width>
      <height>71</height>
     </rect>
    </property>
    <property name="text">
     <string>Print</string>
    </property>
   </widget>
  </widget>
  <widget class="QMenuBar" name="menubar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>474</width>
     <height>22</height>
    </rect>
   </property>
  </widget>
  <widget class="QStatusBar" name="statusbar"/>
 </widget>
 <resources/>
 <connections/>
</ui>

main.cpp:

#include "mainwindow.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}