环境：Qt 5.15 + VS2019

方法一：QGraphicsDropShadowEffect

实现方法参考链接：https://blog.csdn.net/goforwardtostep/article/details/99549750
使用此方法添加窗口阴影，会出现警告信息：

且窗口最大化与还原切换时会出现子控件刷新问题：

方法二：九宫格贴图

实现方法参考链接：https://blog.csdn.net/goforwardtostep/article/details/99549750
需要美工

方法三：paintEvent

实现方法参考链接：https://blog.csdn.net/goforwardtostep/article/details/99549750
此方法绘制圆角边框不够完美
下图右边为paintEvent使用addRoundedRect实现，左边为结合九宫格思想通过paintEvent实现

下面为上图右侧实现代码：
app_window.h

#ifndef APP_WINDOW_H
#define APP_WINDOW_H#include <QWidget>namespace Ui {
class AppWindow;
}class AppWindow : public QWidget
{Q_OBJECTpublic:explicit AppWindow(QWidget *parent = nullptr);~AppWindow();protected:void paintEvent(QPaintEvent* event) override;private:Ui::AppWindow *ui;//阴影宽度int shadowWidth_ = 15;//窗口圆角int windowRadius_ = 9;
};#endif // APP_WINDOW_H

app_window.cpp

#include "app_window.h"
#include "ui_app_window.h"
#include <QPainter>
#include <QPainterPath>AppWindow::AppWindow(QWidget *parent): QWidget(parent), ui(new Ui::AppWindow)
{ui->setupUi(this);setAttribute(Qt::WA_TranslucentBackground); // 背景透明setWindowFlags(Qt::Window | Qt::FramelessWindowHint);QString qss = QString("background-color: rgb(255, 255, 255); border-radius:%1px").arg(windowRadius_);ui->widgetMain->setStyleSheet(qss);ui->gridLayout->setMargin(shadowWidth_);
}AppWindow::~AppWindow()
{delete ui;
}void AppWindow::paintEvent(QPaintEvent *event)
{int radius = shadowWidth_ + windowRadius_;int width = this->width();int height = this->height();QPainter painter(this);painter.setRenderHint(QPainter::Antialiasing, true); //抗锯齿painter.setPen(Qt::NoPen);//线性渐变QLinearGradient linearGradient;linearGradient.setColorAt(0,QColor(255,0,0, 250));linearGradient.setColorAt(1,QColor(255,0,0, 50));//圆形渐变QRadialGradient radialGradient;radialGradient.setColorAt(0,QColor(255,0,0, 250));radialGradient.setColorAt(1,QColor(255,0,0, 50));//左上角radialGradient.setCenter(radius, radius); //中心点radialGradient.setRadius(radius); //半径radialGradient.setFocalPoint(radius, radius); //焦点painter.setBrush(radialGradient);QRectF rectf(0, 0, radius*2, radius*2);QPainterPath path;path.moveTo(radius, radius);//移动圆心path.arcTo(rectf, 90, 90);painter.drawPath(path);   //画路径(扇形)//左边linearGradient.setStart(radius, height/2);linearGradient.setFinalStop(0, height/2);painter.setBrush(linearGradient);path.clear();path.addRect(0, radius, radius, height - radius*2);painter.drawPath(path);//左下角radialGradient.setCenter(radius, height - radius); //中心点radialGradient.setRadius(radius); //半径radialGradient.setFocalPoint(radius, height - radius); //焦点painter.setBrush(radialGradient);path.clear();path.moveTo(radius, height - radius);//移动圆心rectf.setRect(0, height - radius*2, radius*2, radius*2);path.arcTo(rectf, 180, 90);painter.drawPath(path);   //画路径(扇形)//下边linearGradient.setStart(width/2, height - radius);linearGradient.setFinalStop(width/2, height);painter.setBrush(linearGradient);path.clear();path.addRect(radius, height - radius, width - radius*2, radius);painter.drawPath(path);//右下角radialGradient.setCenter(width - radius, height - radius); //中心点radialGradient.setRadius(radius); //半径radialGradient.setFocalPoint(width - radius, height - radius); //焦点painter.setBrush(radialGradient);path.clear();path.moveTo(width - radius, height - radius);//移动圆心rectf.setRect(width - radius*2, height - radius*2, radius*2, radius*2);path.arcTo(rectf, 270, 90);painter.drawPath(path);   //画路径(扇形)//右边linearGradient.setStart(width - radius, height/2);linearGradient.setFinalStop(width, height/2);painter.setBrush(linearGradient);path.clear();path.addRect(width - radius, radius, radius, height - radius*2);painter.drawPath(path);//右上角radialGradient.setCenter(width - radius, radius); //中心点radialGradient.setRadius(radius); //半径radialGradient.setFocalPoint(width - radius, radius); //焦点painter.setBrush(radialGradient);path.clear();path.moveTo(width - radius, radius);//移动圆心rectf.setRect(width - radius*2, 0, radius*2, radius*2);path.arcTo(rectf, 0, 90);painter.drawPath(path);   //画路径(扇形)//上边linearGradient.setStart(height/2, radius);linearGradient.setFinalStop(height/2, 0);painter.setBrush(linearGradient);path.clear();path.addRect(radius, 0, width - radius*2, radius);painter.drawPath(path);
}