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May be related to https://bugzilla.libsdl.org/show_bug.cgi?id=4859 because as I see you already started working on a patch.

In SDL_Video.c file there is

        if ((want != have) && (_this->SetWindowResizable)) {
            if (want) {
                window->flags |= SDL_WINDOW_RESIZABLE;
            } else {
                window->flags &= ~SDL_WINDOW_RESIZABLE;
            }
            _this->SetWindowResizable(_this, window, (SDL_bool) want);
        }

so that SDL_WINDOW_RESIZABLE flag is set only if there is _this->SetWindowResizable. 

Currently Wayland backend doesn't have this function defined. I made a simple fix:

        if ((want != have)) {
            if (want) {
                window->flags |= SDL_WINDOW_RESIZABLE;
            } else {
                window->flags &= ~SDL_WINDOW_RESIZABLE;
            }
            (_this->SetWindowResizable)
                _this->SetWindowResizable(_this, window, (SDL_bool) want);
        }

and resizing works fine for me on KDE and also on Gnome after I set the hittest function. I'm not sure if/how it affects other backends though.

Or in worst case maybe just create empty SetWindowResizable function for wayland backend to make sure it doesn't break anything else?