效果如图所示：

创建新类MovablePoint，继承自QWidget.

MovablePoint头文件:

#ifndef MOVABLEPOINT_H
#define MOVABLEPOINT_H

#include <QWidget>
#include <QPainter>
#include <QPaintEvent>
#include <QStyleOption>
#include <QMouseEvent>

class MovablePoint : public QWidget
{
    Q_OBJECT
public:
    explicit MovablePoint(QWidget *parent = nullptr);

public:
    int      radius;
    bool    mouse_pressed;
    QPoint pressed_pos;
    QPoint previous_pos;
    QPoint current_pos;

protected:
    void paintEvent(QPaintEvent*);
    void mousePressEvent(QMouseEvent*);
    void mouseReleaseEvent(QMouseEvent*);
    void mouseMoveEvent(QMouseEvent*);
};

#endif // MOVABLEPOINT_H

MovablePoint.cpp:

#include "movablepoint.h"

MovablePoint::MovablePoint(QWidget *parent)
    : QWidget{parent}
{
    mouse_pressed = false;

    radius = 5;
    this->setFixedSize(2*radius + 1,2*radius + 1);

    //this->setCursor(Qt::SizeAllCursor);

    setAttribute(Qt::WA_TranslucentBackground);

    this->setStyleSheet("QWidget{background-color: blue;border-radius:5px;}");
}

void MovablePoint::mouseMoveEvent(QMouseEvent *event)
{
    if (mouse_pressed){
        QPoint _cur_pos = this->mapToGlobal(event->pos());
        QPoint _off        =  _cur_pos - previous_pos;

        QRect _rect = this->geometry();
        _rect.moveTopLeft(_rect.topLeft() + _off);
        this->setGeometry(_rect);

        previous_pos = _cur_pos;
    }
}

void MovablePoint::mouseReleaseEvent(QMouseEvent* event)
{
    mouse_pressed = false;
}

void MovablePoint::mousePressEvent(QMouseEvent *event)
{
    mouse_pressed = true;
    previous_pos = this->mapToGlobal(event->pos());

}

void MovablePoint::paintEvent(QPaintEvent*)
{
    QStyleOption opt;
    opt.init(this);
    QPainter p(this);
    style()->drawPrimitive(QStyle::PE_Widget, &opt, &p, this);
}

为了使部件变成圆形：

第一 设置了背景透明及边缘半径：

    setAttribute(Qt::WA_TranslucentBackground);

    this->setStyleSheet("QWidget{background-color: blue;border-radius:5px;}");

第二 在paintEvent中重绘部件

    QStyleOption opt;
    opt.init(this);
    QPainter p(this);
    style()->drawPrimitive(QStyle::PE_Widget, &opt, &p, this);

【免费】QT可移动点类及其测试程序资源-CSDN文库