to be more consistent with other classes, let's name the new files worksheetcontrolitem.h/cpp and the class WorksheetControlItem.

spaces around the operators are not consistently used in Cantor's code nor in this patch. Let's consistently use spaces around every operator.

unnecessary blank.

space in front of :

space if front of :

space in front of the opening bracket (

function names start with a small letter.

the debug output below doesn't see to be relevant anymore.

similar to how we visualize the selection of the control item we should also visualize the item when it's being hovered. Let's introduce a new variable m_overed here and handle it appropriately in hoverEnterEvent() and in hoverLeaveEvent().

painter->fillRect(rect(), QApplication::palette().color(QPalette::Highlight) would also do the job I think.

this assignment and the evaluation of buttonDownPos() is only needed if we have Qt::LeftButton. I'd use

if (event->buttons() != Qt::LeftButton)
    return;

const QPointF downPos = event->buttonDownPos(Qt::LeftButton);
if (contains(downPos) && (event->pos() - downPos).manhattanLength() >= QApplication::startDragDistance())
{
...
}

for QPointF it's better/faster to work with object copies and not with references.

inconsistent usage of spaces here.

these is* variables sound like function names. Let's consistently name private variables m_* like in WorksheetTextItem, etc.

Where exactly?

Well, but this variables is public, so, I think, we could say, that they part of public interface.