QMediaPlayer setVideoOutput locks up user interface

[gmparker2000]

I took the mdi example from qt examples as a starting point. I created my own mdi child window called "VideoMdiChild".

VideoMdiChild *MainWindow::createMdiChild()
{
    VideoMdiChild *child = new VideoMdiChild;
    mdiArea->addSubWindow(child);
    return child;
}

This video mdi child uses a form that contains a video widget. The video widget contains a QMediaPlayer. When I open the video mdi child it locks up the entire user interface until I resize the window. Once resized, the menu becomes usable again. The toolbar is also unresponsive until I tear it off and place it somewhere else.

If I remove the line "_mediaPlayer->setVideoOutput(videoWidget);" from the following code, then things work properly. Of course this means that the video is not rendered.

VideoWidget::VideoWidget(QWidget *parent) :
    QWidget(parent)
{
    _mediaPlayer = new QMediaPlayer(
                this
                , QMediaPlayer::VideoSurface);

    QVideoWidget *videoWidget = new QVideoWidget;
    _mediaPlayer->setVideoOutput(videoWidget);

    QBoxLayout *layout = new QVBoxLayout;
    layout->addWidget(videoWidget);
    setLayout(layout);

   ...
}

Am I doing something obviously wrong? I tried 5.11.2, 5.11.1, and 5.9.7. They all exhibit the same behavior.

Qt 5.11.2 (x86_64-little_endian-lp64 shared (dynamic) release build; by GCC 5.3.1 20160406 (Red Hat 5.3.1-6)) on "xcb"
OS: Ubuntu 18.04.1 LTS [linux version 4.15.0-42-generic]

Architecture: x86_64; features: SSE2 SSE3 SSSE3 SSE4.1 SSE4.2 AVX AVX2

[gmparker2000]

So it looks like it turned out to be related to display scaling in KDE. If you set the scale to an even number like 2 then there are no problems. I had changed the scaling to 1.7. I have it set to 2 now and the problem appears to be gone.

[SGaist]

Hi,

Do you have the same behavior if you use your distribution provided Qt ?

[gmparker2000]

So it looks like it turned out to be related to display scaling in KDE. If you set the scale to an even number like 2 then there are no problems. I had changed the scaling to 1.7. I have it set to 2 now and the problem appears to be gone.

[SGaist]

Thanks for sharing your findings !