Issue with drag and drop

[Cobra91151]

@Christian-Ehrlicher

I tried using a small example program but still the same issue is present. It does not call any of drag/drop events. This time I used the Qt 5.9.9, Qt 5.15.9 and Qt 6.4.3.

DragAndDropExample.pro:

QT       += core gui

greaterThan(QT_MAJOR_VERSION, 4): QT += widgets

CONFIG += c++17

# You can make your code fail to compile if it uses deprecated APIs.
# In order to do so, uncomment the following line.
#DEFINES += QT_DISABLE_DEPRECATED_BEFORE=0x060000    # disables all the APIs deprecated before Qt 6.0.0

CONFIG(debug, debug|release) {
    DESTDIR = $$OUT_PWD/debug
} else {
    DESTDIR = $$OUT_PWD/release
}

MOC_DIR = ./resources/moc
OBJECTS_DIR = ./resources/objects
RCC_DIR = ./resources/resource

SOURCES += \
    appqframe.cpp \
    main.cpp \
    mydialog.cpp

HEADERS += \
    appqframe.h \
    mydialog.h

# Default rules for deployment.
#qnx: target.path = /tmp/$${TARGET}/bin
#else: unix:!android: target.path = /opt/$${TARGET}/bin
#!isEmpty(target.path): INSTALLS += target

AppQFrame.h:

#ifndef APPQFRAME_H
#define APPQFRAME_H

#include <QWidget>
#include <QFrame>
#include <QMouseEvent>
#include <QEvent>
#include <QMimeData>
#include <QDebug>
#include <QDrag>
#include <QDragMoveEvent>
#include <QDragEnterEvent>
#include <QDropEvent>

class AppQFrame : public QFrame
{
    Q_OBJECT
public:
    explicit AppQFrame(QWidget *parent);

signals:
    void mouseMove(QMouseEvent *event);
    void mouseClick(QMouseEvent *event);
    void mouseRelease(QMouseEvent *event);
    void mouseEnter();
    void mouseLeave();

protected:
    void mouseMoveEvent(QMouseEvent *event) override;
    void mousePressEvent(QMouseEvent *event) override;
    void mouseReleaseEvent(QMouseEvent *event) override;
    bool event(QEvent *event) override;
    void dragMoveEvent(QDragMoveEvent *event) override;
    void dragEnterEvent(QDragEnterEvent *event) override;
    void dropEvent(QDropEvent *event) override;
};

#endif // APPQFRAME_H

AppQFrame.cpp:

#include "appqframe.h"

AppQFrame::AppQFrame(QWidget *parent) : QFrame(parent)
{

}

void AppQFrame::mouseMoveEvent(QMouseEvent *event)
{
    emit mouseMove(event);
    QWidget::mouseMoveEvent(event);
}

void AppQFrame::mousePressEvent(QMouseEvent *event)
{
    emit mouseClick(event);
    QWidget::mousePressEvent(event);
}

void AppQFrame::mouseReleaseEvent(QMouseEvent *event)
{
    emit mouseRelease(event);
    QWidget::mouseReleaseEvent(event);
}

bool AppQFrame::event(QEvent *event)
{
    switch (event->type()) {
        case QEvent::HoverEnter:
             emit mouseEnter();
            break;
        case QEvent::HoverLeave:
             emit mouseLeave();
            break;
        case QEvent::DragEnter:
             qDebug() << "Drag enter!!!";
            break;
        case QEvent::DragLeave:
             qDebug() << "Drag leave!!!";
            break;
        case QEvent::DragMove:
             qDebug() << "Drag move!!!";
            break;
        case QEvent::DragResponse:
             qDebug() << "Drag response!!!";
            break;
        case QEvent::Drop:
             qDebug() << "Drop!!!";
            break;
        case QEvent::AcceptDropsChange:
             qDebug() << "Accept Drops Change!!!";
            break;
        default:
            break;
    }

    return QWidget::event(event);
}

void AppQFrame::dragMoveEvent(QDragMoveEvent *event)
{
    qDebug() << "dragMoveEvent: " << event->mimeData()->hasUrls();

    if (event->mimeData()->hasUrls()) {
        event->acceptProposedAction();
        event->accept();
    }
}

void AppQFrame::dragEnterEvent(QDragEnterEvent *event)
{
    qDebug() << "dragEnterEvent: " << event->mimeData()->hasUrls();

    if (event->mimeData()->hasUrls()) {
        event->acceptProposedAction();
        event->accept();
    }
}

void AppQFrame::dropEvent(QDropEvent *event)
{
    qDebug() << "dropEvent: " << event->mimeData()->urls();
    event->setDropAction(Qt::CopyAction);

    foreach (const QUrl &url, event->mimeData()->urls()) {
        QString fileName = url.toLocalFile();
        qDebug() << "Dropped file:" << fileName;
    }
}

main.cpp

#include <QApplication>
#include "mydialog.h"

int main(int argc, char *argv[])
{
    QApplication a(argc, argv);
    MyDialog w;
    w.show();
    return a.exec();
}

mydialog.h

#ifndef MYDIALOG_H
#define MYDIALOG_H

#include <QDialog>
#include <QHBoxLayout>
#include "appqframe.h"

class MyDialog : public QDialog
{
    Q_OBJECT

public:
    MyDialog(QWidget *parent = nullptr);
    ~MyDialog();
};
#endif // MYDIALOG_H

mydialog.cpp

#include "mydialog.h"

MyDialog::MyDialog(QWidget *parent)
    : QDialog(parent)
{
    setWindowTitle(QObject::tr("Feedback"));
    setMinimumSize(480, 610);
    setWindowModality(Qt::ApplicationModal);
    setWindowFlags(Qt::Dialog | Qt::MSWindowsFixedSizeDialogHint | Qt::WindowCloseButtonHint);
    setAcceptDrops(true);
    AppQFrame *attachmentsFrame = new AppQFrame(this);
    attachmentsFrame->setFixedSize(550, 390);
    attachmentsFrame->setObjectName("inputItem");
    attachmentsFrame->setStyleSheet("border: 0.5px dashed #000000; margin: 10px;");
    attachmentsFrame->setAcceptDrops(true);
    attachmentsFrame->setMouseTracking(true);
    attachmentsFrame->setAttribute(Qt::WA_Hover);
    attachmentsFrame->setVisible(true);
    QHBoxLayout *testLayout = new QHBoxLayout();
    testLayout->addWidget(attachmentsFrame);
    setLayout(testLayout);
}

MyDialog::~MyDialog()
{
}

Screenshot:

It only checks for QEvent::AcceptDropsChange event. Any ideas why it can not accept drag and drop? Thanks.

[JonB]

@Cobra91151
A couple of thoughts.

One tiny thing to rule out: Windows itself does not allow drag/drop between applications running elevated and those not. Make sure both Explorer and your application are running as the same, non-elevated user?

Second I would try reducing your application to the bare minimum to test. Get rid of QDialog, get rid of that one's setAcceptDrops(true), get rid of setWindowModality(Qt::ApplicationModal), get rid of your AppQFrame. A simple (sub-classed) QWidget, with nothing in it, should be enough to test the basics. After that, it's a bit long, but I think the example at https://wiki.qt.io/Drag_and_Drop_of_files should be copyable for you to test?

[Cobra91151]

@JonB

I have tried it on Windows 11, Windows 10, Windows 8.1 and Windows 7 VMs and on my laptop (Windows 10) still the same issue exists.

Now, I will try your suggestions and reply. Thanks.

[Cobra91151]

@JonB

I tried a simple sub-classed QWidget but it still does not work. For some reason it blocks the drag and drop, no debug logs from these events, nothing is printed to the console. Only mouse events and QEvent::AcceptDropsChange is printed.

I have built Qt by my own, I think some library which is required for drag and drop is missing on my system. I will try the official Qt build from Maintenance Tool to verify how it works.

[JonB]

@Cobra91151
OK, I don't know. Did you actually copy/try that https://wiki.qt.io/Drag_and_Drop_of_files as-is so we know the code should be as they intend?

[Cobra91151]

@JonB

I tried official Qt build with this small example, still the same issue. https://wiki.qt.io/Drag_and_Drop_of_files It does not call any of drag or drop events. It should call the drag/drop events without any code just print it to console using qDebug?

setAcceptDrops(true);
qDebug() << "acceptDrops: " << acceptDrops();

Returns:
acceptDrops: true

But it still does not allow to drop anything, the cursor is blocking, which means it ignores the drag and drop operation even though I set it to setAcceptDrops.
I think, it should be activated somewhere to accept drops? Or why it blocks the drag and drop?

[Cobra91151]

@JoeCFD @Christian-Ehrlicher @JonB

Finally, I found the root of this issue. The issue is with administrative privileges. When my program has administrative privileges then for some reason the drag and drop operation fails and Qt widget does not accept it and ignores it. When I remove the admin privileges, then it works well.

Log:

dragEnterEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dragMoveEvent:  true
dropEvent:  (QUrl("file:///C:/Users/cobra/Documents/Projects/CPP/build-DragAndDropExample-Desktop_Qt_5_15_2_MSVC2019_32bit-Debug/debug/opengl32sw.dll"))
Dropped file: "C:/Users/cobra/Documents/Projects/CPP/build-DragAndDropExample-Desktop_Qt_5_15_2_MSVC2019_32bit-Debug/debug/opengl32sw.dll"

I have found the similar issue here:
https://forum.qt.io/topic/131155/qt-applications-cannot-drag-and-drop-files-when-running-under-windows-with-administrator-rights/13

For example, I need my program to have admin privileges and allow the drag and drop. Now, the question is how to fix such issue? Thank you.

[Christian Ehrlicher]

@Cobra91151 said in Issue with drag and drop:

Finally, I found the root of this issue. The issue is with administrative privileges. When my program has administrative privileges then for some reason the drag and drop operation fails and Qt widget does not accept it and ignores it.

@JonB already told you this some time ago:

One tiny thing to rule out: Windows itself does not allow drag/drop between applications running elevated and those not. Make sure both Explorer and your application are running as the same, non-elevated user?

[Cobra91151]

@Christian-Ehrlicher said in Issue with drag and drop:

@JonB already told you this some time ago:

One tiny thing to rule out: Windows itself does not allow drag/drop between applications running elevated and those not. Make sure both Explorer and your application are running as the same, non-elevated user?

So, how to allow drag/drop between application running elevated and non-elevated user? Is it possible using Win API and Qt? Why Qt just ignores it and not set the debug message: "this kind of drag/drop opration is not allowed"? Thanks.

[Christian Ehrlicher]

@Cobra91151 said in Issue with drag and drop:

Why Qt just ignores it and not set the debug message: "this kind of drag/drop opration is not allowed"?

Because, as already said, a drag and drop between apps with different privileges is not allowed - it's not a program thing but windows - so how should the (possible) receiver print something when it does not get notified in any way?

There is nothing Qt can do against it. If you find a way feel free to report it.

[JonB]

@Cobra91151 said in Issue with drag and drop:

have admin privileges and allow the drag and drop

Google Windows have admin privileges and allow the drag and drop if you want to read up about it, e.g. https://jacquesheunis.com/post/dragndrop-admin/

It turns out that if you run something “as admin” in Windows, you won’t be able to drag-and-drop files into that program from somewhere else that wasn’t run as admin. This is to prevent so-called “Shatter attacks” which could result in privilege escalation

Maybe you will find some "hacks", I don't know. https://www.reddit.com/r/discordapp/comments/l63doo/cant_drag_and_drop_while_running_as_admin/ claims

Okay, I just found the fix for it.

HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Windows\CurrentVersion\Policies\System

and change EnableLUA value to 0

(Is that disabling UAC to get around it??) Maybe that works and you only care about your own machine?

In a word, you're not supposed to do this. Maybe you can make your program not require admin privileges, maybe you could delegate that to a non-UI program you communicate with.

[Cobra91151]

@JonB

No need to change the registry values. I have found hack with Notepad (admin privileges) which allows to drap/drop files directly from Windows Explorer to my program with admin privileges. Additionally, I can drop files from another program like Total Commander (with admin privileges) or mine program with admin privileges which has QFileSystemModel (I called it PC Explorer).

Screenshot:

[JonB]

@Cobra91151 said in Issue with drag and drop:

No need to change the registry values. I have found hack with Notepad (admin privileges) which allows to drap/drop files directly from Windows Explorer to my program with admin privileges

LOL :)