Waylandsink with Qt QWidget

[Terry Barnaby]

I am trying to output a video stream into a Qt QWidget using the 
waylandsink plugin. This is with gstreamer 1.20 and QT 5.15.2.

In general this is working, but if the QWidget gets hidden as the result 
of being in something like a QStackedWindow and then raised the video is 
no longer shown.

I am using the Weston compositor on an embedded platform, but I have 
also tried the KDE compositor under Fedora35 with the same effect.

It may be related to the issue I saw in 
https://bugs.kde.org/show_bug.cgi?id=445631 where waylandsink's 
subsurface is not drawn as its parent (QWidget) surface remains in a 
"hidden" state.

I'm not sure which of the Qt, waylandsink, Wayland compositor is really 
at fault here, but I assume that either waylandsink, which has access to 
all of the surfaces and subsurfaces, could "wake up" the QWidget's 
surface when it wants to draw or perhaps better the QWidget should 
somehow "wakeup" it's surface when it is raised.

Any ideas ?
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