How to make svg painters measure text correctly

[ceraolo]

I'm writing a program to create scientific plots on screen, and to produce Scalable Vector Graphics (SVG) copies on the hard disk.
I came across an issue on SVG's. To show it I've created the following code.
I know that to create the SVG in the paintEvent method is silly, but I just wanted to show the issue in a very compact code.

#include "mainwindow.h"
#include <QPainter>
#include <QSvgGenerator>
#include <QMainWindow>
 
MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
}
 
MainWindow::~MainWindow()
{
 
}
void MainWindow::paintEvent(QPaintEvent *){
  int xPos;
  QPainter p;
  QRect r;
  QString s="This is some significant text";
 
  //Writing on MainWindow:
  p.begin(this);
  p.drawText(0,0,0,0,0,s,&r);
  p.drawText(r,s);
  p.drawRect(r);
 
  p.drawText(0,40,s);
  QFontMetrics m=p.fontMetrics();
  xPos=m.width(s);
  p.drawLine(QPoint(xPos,40),QPoint(xPos,40-m.height()));
  p.end();
 
  //Creating and writing on the svg:
  QSvgGenerator generator;
  generator.setFileName("svgIssue.svg");
  p.begin(&generator);
  p.drawText(0,0,0,0,0,s,&r);
  p.drawText(r,s);
  p.drawRect(r);
 
  p.drawText(0,40,s);
  QFontMetrics m1=p.fontMetrics();
  xPos=m1.width(s);
  p.drawLine(QPoint(xPos,40),QPoint(xPos,40-m.height()));
  p.end();
}

#include "mainwindow.h"
#include <QPainter>
#include <QSvgGenerator>
#include <QMainWindow>

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
{
}

MainWindow::~MainWindow()
{
    
}
void MainWindow::paintEvent(QPaintEvent *){
  int xPos;
  QPainter p;
  QRect r;
  QString s="This is some significant text";

  //Writing on MainWindow:
  p.begin(this);
  p.drawText(0,0,0,0,0,s,&r);
  p.drawText(r,s);
  p.drawRect(r);

  p.drawText(0,40,s);
  QFontMetrics m=p.fontMetrics();
  xPos=m.width(s);
  p.drawLine(QPoint(xPos,40),QPoint(xPos,40-m.height()));
  p.end();

  //Creating and writing on the svg:
  QSvgGenerator generator;
  generator.setFileName("svgIssue.svg");
  p.begin(&generator);
  p.drawText(0,0,0,0,0,s,&r);
  p.drawText(r,s);
  p.drawRect(r);

  p.drawText(0,40,s);
  QFontMetrics m1=p.fontMetrics();
  xPos=m1.width(s);
  p.drawLine(QPoint(xPos,40),QPoint(xPos,40-m.height()));
  p.end();
}

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This is what I see on screen (i.e. it is correct)
winScreen.jpg

And this is what I see when I browse the svg file using google chrome (i.e. the right part of the rectangle and the vertical bar are misplaced:
svgToChrome.jpg

I.e. my software computes correctly the text width on screen, but DOES NOT inside the SVG.
You can imagine what such an error can cause on a program that creates scientific plots!

Can anyone suggest a workaround for this issue?
Thanks a lot in advance.
MC