The documentation for QWidget::inputContext() states that by default the context is inherited from the widget's parent, in reality if no input context has been set on the widget the context is inherited from QApplication even if a context has been set on a parent item. So in the following snippet child.inputContext() will return qApp->inputContext() not parent.inputContext().

QWidget parent;
parent.setInputContext(new InputContext);

QWidget child(&parent);
child.setAttribute(Qt::WA_InputMethodEnabled, true);

This is an issue with setting an input context on a QGraphicsView for example because the widget returned by QGraphicsSceneEvent is the view's viewport not the view itself.