• Compile and run the code below

#include <QWidget>
#include <QApplication>
#include <QDebug>
#include <QMouseEvent>

class W : public QWidget
{
  void mousePressEvent(QMouseEvent* e)
  {
    qDebug()<<"mousePressEvent"<<e;
  }
  void mouseMoveEvent(QMouseEvent* e)
  {
    qDebug()<<"mouseMove"<<e;
  }
  void mouseReleaseEvent(QMouseEvent* e)
  {
    qDebug()<<"mouseReleaseEvent"<<e;
  }

};

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

  W w;

  w.show();

  return a.exec();
}

• press and drag the mouse around. "mouseMove" is printed continuously.
  When you move over the titlebar you will start receiving a stream of mousePressEvent-s instead!

The bug is surprisingly evil in real life because it breaks almost all mouse interaction which required dragging - content dragging (navigating), widgets dragging ("sliders"), Drag-and-Drop - all.

Debugging

Problem is QGuiApplicationPrivate::processMouseEvent receives both
QWindowSystemInterfacePrivate::FrameStrutMouse and QWindowSystemInterfacePrivate::Mouse for the same event!
(The Strut is received second, breaking the normal processing of Mouse, most notably because it's buttons are empty, which triggers a special case in processMouseEvent)