QApplication in non-main thread

AboveL

Hi!

I'm trying to write a lib based on Qt that'd have QApplication running in a separate thread. The idea is to have these exported functions (to be called from "user's" thread):

Init(), that'd create the new thread (in which QApplication is created, ::exec() is called and QApplication is deleted after ::exec() returned),
Exit(), that would emit a signal connected to QApplication's quit slot, and then join the created thread.
Other functions that would only be called in between the above two, create some QObjects and interact with the QApplication instance. All the created QObjects will be deleted before QApplication::exec() returns.

Unfortunately I have to use a rather old Qt version (5.5.0), in which I experience some clean-up issues.
The problem is that there are still some QObjects alive after Exit() returned - I see that by setting a breakpoint at QObjectPrivate::~QObjectPrivate() which gets triggered several times when my main returns. Another problem is that if some secondary event loops were created in between Init() and Exit() calls, I also get this warning when main returns:

QObject::~QObject: Timers cannot be stopped from another thread

Here's a code that reproduces the problem for me (based on Why the main thread is needed for QCoreApplication?):

#include <QApplication>
#include <QTimer>
#include <QMessageBox>
#include <thread>
void func()
{
    QMessageBox::information(nullptr, "Info", "Hello!");
}
void runsInOtherThread(int argc, char *argv[])
{
    QApplication a(argc, argv);
    a.setQuitOnLastWindowClosed(false);
    QTimer::singleShot(100, &a, &func);
    QTimer::singleShot(3000, &a, &QCoreApplication::quit); //spin for 3 seconds and quit
    a.exec();
}

int main(int argc, char *argv[])
{
    std::thread t(runsInOtherThread, argc, argv);
    // do some stuff while Qt runs in another thread
    // ...
    t.join();
    return 0;
}

If I comment out the QMessageBox part, I don't get the warning but still see some QObjects destroyed at main's exit. On Qt 5.10 it works fine though (no QObjectPrivate::~QObjectPrivate() calls after the thread was joined).

Can there be some manual clean-up used for 5.5 to avoid this issue? Or maybe some simple patch to Qt?