This has been reported to the the SDL bugtracker as well but there's talk about how this is not SDL's problem. The bug report is here: bugzilla.libsdl.org/show_bug.cgi?id=2710 Reproducible: Always
I built the hello_world.c attached over there and ran it with SDL_VIDEODRIVER=wayland on kwin master and can confirm no decos (I don't know why). It does get a deco when SDL uses Xwayland.
Wayland uses client-side decorations by default. Of course there are no decorations, that's the (stupid) idea of client-side decorations. KWin has an additional interface to negotiate window decorations, so that KWin can add window decorations. So SDL could implement this so that it can get decorations, the protocol is described at: https://quickgit.kde.org/?p=kwayland.git&a=blob&h=8bc106c7c42a40f71dad9a884824a7a9899e7b2f&hb=29d6bbd92a20ced2c1a3301f5df9e605999cf894&f=src%2Fclient%2Fprotocols%2Fserver-decoration.xml And KWayland also has an implementation for it. If SDL does not like the (stupid) default behavior of client-side-decoration I highly recommend them to "complain" on the Wayland developer mailing list. They will have mine and Qt developer's fully support.
Just a quick follow up in case someone finds this bug via Google: SDL supports both the KDE-specific SSD protocol for legacy systems and xdg-decoration for future machines. This will be in SDL 2.0.10 and later. --ryan.
Ryan, just fyi, over in the KWin/Plasma team we've seen your video and activity in the Gnome GitLab issue and were really happy and appreciative to see that got going. Thanks!