To update the value of QScrollBar from a loop If QCoreApplication::flush()is used(to return into event loop) QProgress bar is not refreshed (not drawn at the GUI)

Attached example reproduces this:
Just run the example and hit start, progress bar is shown but it does not progress at all.

if processEvent() is used it works but then processEvent() process even thsoe events which are queued and you dont want that to happen at this moment.