(#z25ur2q) Well, some time ago I put this in my ~/.Xdefaults:

URxvt.keysym.Control-Up:    \033[1;5A
    URxvt.keysym.Control-Down:  \033[1;5B
URxvt.keysym.Control-Left:  \033[1;5D
    URxvt.keysym.Control-Right: \033[1;5C

Probably to behave more like XTerm and fix a few other issues I had with other programs. But, it turns out, tcell expects the original sequence: https://github.com/gdamore/tcell/blob/main/terminfo/r/rxvt/term.go#L487

Hmm.