Linux: Dark window when dragging if compositing is disabled in window manager

[juangburgos]

Hi,

I am using the library in the QUaModbusClient open source project. I have tested on Windows ans CentOS and all fine. I have tried on Lubuntu and it seems there is some issue when dragging over a detached dock into a Layout.

Has something similar happened to anybody? Is it an OS issue or a library issue?

Thanks

[githubuser0xFFFF]

Could you please elaborate what the exact issue is? Is the problem reproducible with the demo application.

Which QT version do you use and what are your global dock manager configuration flags?

[juangburgos]

Hi,

Yes, the issue is reproducible with the latest binary available here (AppImage).

Qt version is 5.12.3. I do not use any specific global dock manager configuration flags, I just ise the library as shipped by the github repo (it is actually included in the project as a Git submodule).

Best,

[githubuser0xFFFF]

I mean, is the problem reproducible with thr ADS demo app.?

[juangburgos]

I would need to deploy it. Will come back to you on that.

[hhslepicka]

@githubuser0xFFFF I can reproduce this issue with the latest code and demo app.
With Linux when dragging the dock I get a black overlay over the whole application. When the dock is docked it goes away.

[githubuser0xFFFF]

It is not a generic Linux issue. I tested it on Ubuntu 19.10 an Kubuntu 18.04 and it works. According to another user this problem occurs, if compositing is deactivated in Linux.

@hhslepicka Which Linux distro do you use?

[githubuser0xFFFF]

@juangburgos Can you please tell me which Lubuntu version you use?

[hhslepicka]

I use RHEL 6 & 7

[iam-peter]

This issue is caused by disabled compositing via the window manager.
https://doc.qt.io/qt-5/qwidget.html#creating-translucent-windows

Creating Translucent Windows
Since Qt 4.5, it has been possible to create windows with translucent regions on window systems that support compositing.
To enable this feature in a top-level widget, set its Qt::WA_TranslucentBackground attribute with setAttribute() and ensure that its background is painted with non-opaque colors in the regions you want to be partially transparent.
Platform notes:
X11: This feature relies on the use of an X server that supports ARGB visuals and a compositing window manager.
Windows: The widget needs to have the Qt::FramelessWindowHint window flag set for the translucency to work.

My current setup is running Xfwm4 as a window manager, because it allows me to easily enable/disable compositing via the following command: xfconf-query -c xfwm4 -p /general/use_compositing -s false.
If compositing is disabled, I get the following result in the demo application when dragging around a DockWidget:

This behavior is caused by the QWidget background set to be translucent.

Qt allows for checking if compositing is running or not with bool QX11Info::isCompositingManagerRunning() documentation
A workaround could utilize the following statement:

#ifdef Q_WS_X11
    if(QX11Info::isCompositingManagerRunning())
        setAttribute(Qt::WA_TranslucentBackground);
#endif

For now I came up with two workarounds for the disabled compositing issue:

1. Use QRegion similar as in the Shaped Clock example to not rely on translucent QWidget. A drawback would be the missing transparency on the drop regions. Maybe it is possible to use a different pattern on the QBrush to achieve a sort of see through look.
2. Use a screenshot of the MainWindow as the background of the DockOverlay. The drawback is that the background would be static as long as the drag operation is ongoing.

[juangburgos]

Sorry, I was AFK on holidays. My experience looks the same as for @iam-peter on Lubuntu 19.10.