I need by QScrollArea to do something specific when the scroll bar appears. I can't find the signal or a method that could help me know when the area is too big and the scroll bar appears.
You might try to install an event filter on the scroll bars of the QScrollArea, and listen for the showEvent. If it is not spontaneous, it came from the application itself. I think that would equate to the QScrollArea showing the widget. There are no signals you can use that I am aware of.