Current implementation in WebCore/platform/graphics/gstreamer/GStreamerGWorld.cpp expects a GStreamer sink that implements the video overlay interface for the fullscreen mode, but GStreamer's waylandsink from gst-plugins-bad does not implement this interface yet
If waylandsink supports the video overlay interface in the future the code in WebCore/platform/graphics/gstreamer/PlatformVideoWindowGtk.cpp should be updated to support Wayland windows (current implementation only supports X).
We removed native fullscreen video display support. Fullscreen display is now the video element itself being scaled up (IIRC). Perhaps this bug can be closed then?
Since we do not depend on the native window system anymore the problem does not exist any more. Closing.