Faced such a problem:
There is a QMdiSubWindow window with widgets hung on it. When you move the cursor to any edge of the inner window, it, as expected, changes the view to two arrows, which means that the window can be resized. However, when you return the cursor to the window itself, the cursor does not change the view back to a regular pointer. But changes when moving the window title bar.
Did someone have that?
UPD: Trial and error method found out that the problem occurs after installing the internal widget

 internalWidget = new QWidget(this); setWidget(internalWidget);

    1 answer 1

    Method "spear" solution was found. I leave here with the note "answer", can someone come in handy.
    The reason was in the connected event filter, more precisely in the return value by default:

     bool CatProductsGUI::eventFilter(QObject *object, QEvent *event) { if(event->type()==QEvent::MouseButtonRelease){ QMouseEvent *me = dynamic_cast<QMouseEvent*>(event); if(object==treeGroups->viewport()){ QModelIndex index = treeGroups->indexAt(me->pos()); if(!index.isValid()){ treeGroups->clearSelection(); } } return true; } return QObject::eventFilter(object, event); //<------------- }

    After replacing QObject with QMdiSubWindow (the immediate ancestor of the window containing the event filter), everything fell into place. Here is such a disagreeable thing.