After running some more tests, I get different results. I never get the:
QEvent::MouseButtonRelease:
OBJECT QComboBoxPrivateContainer::unnamed

When I click a combobox, and release the button (without moving), I get:
QEvent::MouseButtonPress:
OBJECT QComboBox::testbox
and the list expands
Clicking on the same spot (without moving the mouse), I get:
QEvent::MouseButtonRelease:
OBJECT QComboBox::testbox
and the list collapses (no change to selection)

My code:
test.h:
Qt Code: 
  1. #include <QtGui/QtGui>
  2.  
  3. class Test: public QObject
  4. {
  5.   Q_OBJECT
  6. public:
  7.   bool eventFilter(QObject *o, QEvent *e);
  8. };
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test.cpp
Qt Code: 
  1. #include "test.h"
  2. bool Test::eventFilter(QObject *o, QEvent *e)
  3. {
  4.   if (e->type() == QEvent::MouseButtonPress) {
  5.       qDebug() << "QEvent::MouseButtonPress:";
  6.       o->dumpObjectInfo();
  7.   } else if (e->type() == QEvent::MouseButtonRelease) {
  8.       qDebug() << "QEvent::MouseButtonRelease:";
  9.       o->dumpObjectInfo();
  10.   }   if (e->type() == QEvent::KeyPress) {
  11.       qDebug() << "QEvent::KeyPress:";
  12.       o->dumpObjectInfo();
  13.   } else if (e->type() == QEvent::KeyRelease) {
  14.       qDebug() << "QEvent::KeyRelease:";
  15.       o->dumpObjectInfo();
  16.   }
  17.   return false;
  18. }
  19.  
  20. void addit(QObject * o, Test * t)
  21. {
  22.   o->installEventFilter(t);
  23.   foreach (QObject * cobj, o->children())
  24.     cobj->installEventFilter(t);
  25. }
  26.  
  27.  
  28. int main(int argc, char * argv[])
  29. {
  30.   QApplication app(argc, argv);
  31.   QComboBox * box = new QComboBox();
  32.   Test * test = new Test();
  33.   addit(box, test);
  34.   foreach (QObject * cobj, box->view()->children())
  35.     addit(cobj, test);
  36.   box->setObjectName("testbox");
  37.   box->addItem("foo 1");
  38.   box->addItem("foo 2");
  39.   box->addItem("foo 3");
  40.   box->show();
  41.   return app.exec();
  42. }
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to compile:
moc test.h -o test_moc.cpp
g++ test.cpp test_moc.cpp -I /usr/local/Trolltech/Qt4.1.0/include -L /usr/local/Trolltech/Qt4.1.0/lib -lQtGui_debug -lQtCore_debug

I don't understand why the QComboBoxPrivateContainer::unnamed is not shown here.

Greetings,
Beluvius