已纠正写法，谢谢 @Voleking 大佬~
楼歪了，让我扶正过来。目前已实现结果框的边缘大小与company-mode的margin大小一致，即一个字符宽度；两者与高亮行字符对齐一致：
(setq selectrum-display-action '(selectrum-posframe))
(defcustom selectrum-posframe-fringe 1
"The width of selectrum-posframe-fringe."
:type 'number)
(defun posframe-poshandler-point-bottom-left-corner-fringe (info)
"Poshandler looks like company-posframe."
(let* ((parent-window (plist-get info :parent-window))
(point (with-current-buffer (window-buffer parent-window)
(max (line-beginning-position)
(- (plist-get info :position) selectrum-posframe-fringe))))
(info (plist-put info :position-info (posn-at-point point parent-window))))
(posframe-poshandler-point-bottom-left-corner info)))
(defun selectrum-posframe (buffer _alist)
"Function used by `selectrum-display-action."
(frame-root-window
(posframe-show buffer
:min-height 10
:min-width 30
:background-color "#b4eeb4" ;; hl-line background color
:left-fringe (* (frame-char-width) selectrum-posframe-fringe)
:right-fringe (* (frame-char-width) selectrum-posframe-fringe)
:poshandler 'posframe-poshandler-point-bottom-left-corner-fringe
:lines-truncate t
)))
(set-face-attribute 'fringe nil :background 'unspecified :inherit 'hl-line)
(add-hook 'minibuffer-exit-hook 'posframe-delete-all)
TODO：
1、隐藏 “25 M-x”，即 candidates count 和 minibuffer-prompt ；
2、调整“结果框posframe”的宽高使之随candidaes长短多少而变化；
3、fringe背景色与“select-candidate”背景色不一致，可以考虑去掉finge，取而代之的是给每个candidate双侧加上一个字符宽度。
非IT专业，搞不动了……希望各位大佬出手相助 