Show the {what} above the position from 'ballooneval' and close it when the mouse moves. This works like:

let pos = screenpos(v:beval_winnr, v:beval_lnum, v:beval_col)
call popup_create({what}, #{
      \ pos: 'botleft',
      \ line: pos.row - 1,
      \ col: pos.col,
      \ mousemoved: 'WORD',
      \ })

Use {options} to change the properties. See popup_beval_example for an example.

Can also be used as a method:

GetText()->popup_beval({})