Display bug with WA_TranslucentBackground and AA_EnableHighDpiScaling

[EdwinZhou]

I'm building a window desktop app which is frameless and has transparent background. To achieve this, I set WA_TranslucentBackground and transparent FramelessWindowHint in mainWindow. And I also have to support different DPI, so I set AA_EnableHighDpiScaling in main function. It works well when windows system dpi set to 100% or 200%, but it will appear some 1px gap in child widget which is hovered when system dpi set to 125% ,150% , 175% .

image above is normal state main window

image above is bad rendered main window when child widget has beed hovered. You can see 1px transparent gap where arrow point to.

This phenomenon only appears when meet 3 conditions:

1. set QApplication::setAttribute(Qt::AA_EnableHighDpiScaling); in main() function
2. set setAttribute(Qt::WA_TranslucentBackground); setWindowFlags(Qt::FramelessWindowHint); in mainWindow
3. set windows system dpi to 125%, 150%, 175% or some not X00% value。

Could you please tell me how to avoid this gap to be rendered, or how can I achieve both transparent background and supporting different DPI with another solution?
Thanks :)

below is my simple demo to descibe this bug

main.cpp

#include "mainwindow.h"

#include <QApplication>

int main(int argc, char *argv[])
{
    QApplication::setAttribute(Qt::AA_EnableHighDpiScaling);
    QGuiApplication::setHighDpiScaleFactorRoundingPolicy(Qt::HighDpiScaleFactorRoundingPolicy::PassThrough);
    QApplication a(argc, argv);
    MainWindow w;
    w.show();
    return a.exec();
}

mainwindow.h

#ifndef MAINWINDOW_H
#define MAINWINDOW_H

#include <QMainWindow>

QT_BEGIN_NAMESPACE
namespace Ui { class MainWindow; }
QT_END_NAMESPACE

class MainWindow : public QMainWindow
{
    Q_OBJECT

public:
    MainWindow(QWidget *parent = nullptr);
    ~MainWindow();

private:
    Ui::MainWindow *ui;
};
#endif // MAINWINDOW_H

mainwindow.cpp

#include "mainwindow.h"
#include "ui_mainwindow.h"

MainWindow::MainWindow(QWidget *parent)
    : QMainWindow(parent)
    , ui(new Ui::MainWindow)
{
    ui->setupUi(this);

    setAttribute(Qt::WA_TranslucentBackground);
    setWindowFlags(Qt::FramelessWindowHint);
}

MainWindow::~MainWindow()
{
    delete ui;
}

mainwindow.ui

<?xml version="1.0" encoding="UTF-8"?>
<ui version="4.0">
 <class>MainWindow</class>
 <widget class="QMainWindow" name="MainWindow">
  <property name="geometry">
   <rect>
    <x>0</x>
    <y>0</y>
    <width>800</width>
    <height>600</height>
   </rect>
  </property>
  <property name="windowTitle">
   <string>MainWindow</string>
  </property>
  <widget class="QWidget" name="centralwidget">
   <widget class="QWidget" name="widget" native="true">
    <property name="geometry">
     <rect>
      <x>120</x>
      <y>110</y>
      <width>291</width>
      <height>411</height>
     </rect>
    </property>
    <property name="styleSheet">
     <string notr="true">background-color: white; border-radius: 5px</string>
    </property>
    <widget class="QPushButton" name="pushButton">
     <property name="geometry">
      <rect>
       <x>40</x>
       <y>60</y>
       <width>141</width>
       <height>91</height>
      </rect>
     </property>
     <property name="styleSheet">
      <string notr="true">QPushButton{ background-color: gray} QPushButton:hover:!pressed { background-color: #dddddd; }</string>
     </property>
     <property name="text">
      <string/>
     </property>
    </widget>
    <widget class="QPushButton" name="pushButton_2">
     <property name="geometry">
      <rect>
       <x>40</x>
       <y>220</y>
       <width>111</width>
       <height>81</height>
      </rect>
     </property>
     <property name="styleSheet">
      <string notr="true">QPushButton{ background-color: gray} QPushButton:hover:!pressed { background-color: #dddddd; }</string>
     </property>
     <property name="text">
      <string/>
     </property>
    </widget>
   </widget>
  </widget>
  <widget class="QMenuBar" name="menubar">
   <property name="geometry">
    <rect>
     <x>0</x>
     <y>0</y>
     <width>800</width>
     <height>21</height>
    </rect>
   </property>
  </widget>
  <widget class="QStatusBar" name="statusbar"/>
 </widget>
 <resources/>
 <connections/>
</ui>