If you need to send vertices mid-frame, don't use a buffer, it doesn't work. Use immediate mode instead.

[LexiBigCheese]

right now i use this:

use citro3d_sys::{C3D_ImmDrawBegin,C3D_ImmDrawEnd,C3D_ImmSendAttrib};
use ctru_sys::GPU_TRIANGLES;

pub fn imm(f: impl FnOnce()) {
    unsafe {
        C3D_ImmDrawBegin(GPU_TRIANGLES);
    }
    f();
    unsafe {
        C3D_ImmDrawEnd();
    }
}

pub fn attr(xyzw: [f32;4]) {
    unsafe {
        C3D_ImmSendAttrib(xyzw[0],xyzw[1],xyzw[2],xyzw[3]);
    }
}

this is, of course, not sound in the slightest, but could be fixed by some sort of struct that when created, ImmDrawBegin and when dropped, ImmDrawEnd,
The programmer should be informed that the struct needs to be dropped, and a convenience method that takes FnOnce(TheStruct) should be supplied.

[LexiBigCheese]

if you do try to LINEAR the vertices, the vertex and index data will overwrite itself before the GPU even sees it, resulting in a very strange image (when implementing egui, this manifested as having a white screen unless i drew a degenerate triangle, causing portions of the screen to be visible but a simple RGB color, weird graphical glitches in general pretty much. and it did not matter if i was drawArrays or drawElements aka using indices)