Why Widget::grabMouse() installs WH_JOURNALRECORD hook on Windows in Qt 4.8.x

[JakaBac]

In Qt 4.8.x Widget::grabMouse() does this on Windows:

void QWidget::grabMouse()
{
    if (!qt_nograb()) {
        if (mouseGrb)
            mouseGrb->releaseMouse();
        journalRec = SetWindowsHookEx(WH_JOURNALRECORD, (HOOKPROC)qJournalRecordProc, GetModuleHandle(0), 0);
        Q_ASSERT(testAttribute(Qt::WA_WState_Created));
        SetCapture(effectiveWinId());
        mouseGrb = this;
#ifndef QT_NO_CURSOR
        mouseGrbCur = new QCursor(mouseGrb->cursor());
#endif
    }
}

qJournalRecordProc is defined as:

// The procedure does nothing, but is required for mousegrabbing to work
#ifndef Q_WS_WINCE
LRESULT QT_WIN_CALLBACK qJournalRecordProc(int nCode, WPARAM wParam, LPARAM lParam)
{
    return CallNextHookEx(journalRec, nCode, wParam, lParam);
}
#endif //Q_WS_WINCE

I know that Qt 4.8.x is long out of support and Qt 5 does not do this anymore, but...
WH_JOURNALRECORD hooks are subjected to various security requirements since Vista or the SetWindowsHookEx call simply fails.

The thing is, that in some cases it does not fail, but messes up the complete Desktop (until hook is removed by pressing Ctrl+Esc).

If the hook/unhook calls fail or are removed, applications that call grabMouse still work without any noticeable issues.
Since this clearly a hack that was obviously needed to make mouse grabbing working, I would very much like to know why it was done in this way and what was the original problem.

For full implementations of this look here

Thanks a lot!
Jaka

[mrjj]

Hi
You will have to ask the devs on the mailing list.
http://lists.qt-project.org/mailman/listinfo

We are users here and cant answer why it was done that way :)
(we would if we could)

[JakaBac]

Ok, will do.

Thanks!

[JakaBac]

For sake of completeness here is the answer from Qt developers (in case someone reads this and has the same issue):
http://lists.qt-project.org/pipermail/development/2018-April/032630.html

Unfortunately the reason why it was done in that way is still a mystery but it seems that the hook code can be safely removed