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I prepared some working code for reproduce the issue:

main.cpp

#include <opencv/cv.h>
#include <opencv2/opencv.hpp>
#include <thread>

class my_class
{
public:
    my_class();

    void my_function();

private:
    int m_slider;
    std::string m_window;
    cv::Mat m_mat;
};

my_class::my_class() :
    m_slider(0),
    m_window("Separated trackbar pannel")
{
    cv::namedWindow(m_window); //<-- this line freezes app if comment then will be OK
}

void my_class::my_function()
{
    //if place here imshow then this imshow will be last line cause of constructor namedWindow
}

class my_thread_class
{
public:
    my_thread_class();
    void start();
    void stop();

private:
    std::thread m_thread;
    bool m_is_running;
    my_class m_mc;

    void loop();
};

my_thread_class::my_thread_class():
    m_is_running(false)
{
}

void my_thread_class::start()
{
    stop();
    std::this_thread::sleep_for(std::chrono::milliseconds(200));
    m_is_running = true;
    m_thread = std::thread(&my_thread_class::loop, this);
    m_thread.join();
}

void  my_thread_class::stop()
{
    m_is_running = false;
}

void my_thread_class::loop()
{
    cv::Mat frame;
    cv::VideoCapture capture("./video.mp4");
    while(m_is_running)
    {
    capture >> frame;

    if(frame.empty())
    {
        std::cout << "video_capture frame is empty" << std::endl;
        continue;
    }

        m_mc.my_function();

    cv::imshow("TEST", frame); //<-- app stops at this line if namedWindow in constructor
    cv::waitKey(30);
    }
}

int main(int argc, char *argv[])
{
    my_thread_class thread;
    thread.start();

    return 0;
}

Comments in code describe my issue. If comment line cv::namedWindow(m_window); then the video.mp4 will be displayed in window with name "TEST".

Why so?

I prepared some working code for reproduce the issue:

main.cpp

#include <opencv/cv.h>
#include <opencv2/opencv.hpp>
#include <thread>

class my_class
{
public:
    my_class();

    void my_function();

private:
    int m_slider;
    std::string m_window;
    cv::Mat m_mat;
};

my_class::my_class() :
    m_slider(0),
    m_window("Separated trackbar pannel")
{
    cv::namedWindow(m_window); //<-- this line freezes app if comment then will be OK
}

void my_class::my_function()
{
    //if place here imshow then this imshow will be last line cause of constructor namedWindow
}

class my_thread_class
{
public:
    my_thread_class();
    void start();
    void stop();

private:
    std::thread m_thread;
    bool m_is_running;
    my_class m_mc;

    void loop();
};

my_thread_class::my_thread_class():
    m_is_running(false)
{
}

void my_thread_class::start()
{
    stop();
    std::this_thread::sleep_for(std::chrono::milliseconds(200));
    m_is_running = true;
    m_thread = std::thread(&my_thread_class::loop, this);
    m_thread.join();
}

void  my_thread_class::stop()
{
    m_is_running = false;
}

void my_thread_class::loop()
{
    cv::Mat frame;
    cv::VideoCapture capture("./video.mp4");
    while(m_is_running)
    {
    capture >> frame;

    if(frame.empty())
    {
        std::cout << "video_capture frame is empty" << std::endl;
        continue;
    }

        m_mc.my_function();

    cv::imshow("TEST", frame); //<-- app stops at this line if namedWindow in constructor
    cv::waitKey(30);
    }
}

int main(int argc, char *argv[])
{
    my_thread_class thread;
    thread.start();

    return 0;
}

Comments in code describe my issue. If comment line cv::namedWindow(m_window); then the video.mp4 will be displayed in window with name "TEST".

Why so?