see tdf#125219 for qt5/kf5
Steps to reproduce
0) Use qt6 VCL plugin in a Plasma Wayland session
1) open a presentation containing a video in Impress (e.g. attachment 145517 [details] from bug 120452)
2) start slide show by pressing F5
As of now, video is not shown properly. qt5/kf5 use GStreamer's "qwidget5videosink", but that one is not available for Qt 6 and will probably not be ported for qt6. From commit message of the commit that introduced the qt6 VCL plugin:
> commit 88d57cf241209ffec9eaed3e523942ab51af6db6
> Author: Michael Weghorn <email@example.com>
> Date: Wed Sep 29 11:09:51 2021 +0200
> qt6: Add a qt6 VCL plugin
> 6) There's currently no Qt 6 video sink for GStreamer.
> As of today, qt-gstreamer is unmaintained and there is no Qt 6 version.
> The project's README  says:
> > 0. Maintenance Notice
> > ---------------------
> > This code is unmaintained. You can use it at your own risk.
> > If you want to integrate video display in your QML-based UI,
> > you should consider using 'qmlglsink', from gst-plugins-good.
> > This is a well supported video sink that uses the generic
> > gstreamer-gl stack and is in many ways superior to 'qtquick2videosink'
> > that is provided by qt-gstreamer. You can use this code as an example:
> > https://cgit.freedesktop.org/gstreamer/gst-plugins-good/tree/tests/examples/qt/qmlsink
> > If you are not interested in using QML in your UI, then you
> > may use one of the other elements provided by this module
> > (see below). If you do that, it would be helpful to let us
> > know that this code is still useful to you. We may consider
> > adding these elements in one of the core gstreamer modules.
> > If you are here for the Qt-style bindings, I'm sorry to disappoint you.
> > The alternative is to use the C API, or the GStreamermm C++ API.
> > Qt-style bindings are cool, but unfortunately they are very hard
> > to maintain because they are written by hand. If you are interested
> > in continuing this project, you are welcome to implement a
> > generator for them, probably based on GObject-Introspection.
> > I am happy to provide directions if you want to pursue such a thing.
> Therefore, the Qt video sink handling is qt5-only and the
> corresponding handling for GOBJECT (used for the GStreamer
> video sink handling) was not taken over for qt6.
> This presumably means that video playback in Impress presentations
> does not work when using qt6 with they Qt Wayland plugin,
> s. tdf#125219 for the corresponding bug for qt5/kf5.
> (I did not build the qtwayland module to actually test this, though.
> Video playback with the Qt xcb plugin in a Wayland session works.)
>  https://doc-snapshots.qt.io/qt6-dev/extras-changes-qt6.html
>  https://cgit.freedesktop.org/gstreamer/qt-gstreamer/tree/README
2 potential ways forward:
1) Add support for GStreamer's waylandsink, similar to how this commit did for gtk3:
Author: Caolán McNamara <firstname.lastname@example.org>
Date: Thu May 23 17:09:51 2019 +0100
tdf#125271 under wayland without gtksink, try waylandsink
2) Maybe it makes sense to take a look whether implementing an own XPlayer using Qt's own video/media framework  makes sense, which uses GStreamer internally. The waylandsink approach e.g. has the drawback that Qt private API is needed to get a handle to the Wayland surface.
(In reply to Michael Weghorn from comment #1)
> 2) Maybe it makes sense to take a look whether implementing an own XPlayer
> using Qt's own video/media framework  makes sense, which uses GStreamer
> internally. The waylandsink approach e.g. has the drawback that Qt private
> API is needed to get a handle to the Wayland surface.
>  https://doc.qt.io/qt-5/videooverview.html
May be just revert deleted VLC embedded feature https://gerrit.libreoffice.org/c/core/+/108283 and to complete its implementing?
(In reply to Roman Kuznetsov from comment #2)
> May be just revert deleted VLC embedded feature
> https://gerrit.libreoffice.org/c/core/+/108283 and to complete its
Thanks, that might be another option to consider.