Comment by MisterTea

FWIW, Plan 9 windows work this way. They are just plain UTF-8 buffers with no typewriter emulation. You can edit any text you wish. If you want graphics, the draw device is a 2D graphics compositor you load assets into then issue rendering commands. Text is a draw primitive and easily displayed any way you want, angled, rotated, moving around, colorful. VT emulation is accomplished by vt(1) which does VT over stdio and emulates an ANSI typewriter using the draw device and it works well. You could even write a Plan 9 native TUI That way, just run it under vt(1). But I would not recommend that - go native.