On macOS overlay-arrow is not draw on top of bitmaps already present
in the fringe, like its done on X Window System (src/xterm.c).  This is
to the detriment of gdb-mi.el where the overlay-arrow hides the
breakpoint when they exist on the same row.

It can be reproduced with the following:
(require 'gdb-mi)
(defvar overlay-arrow (make-marker))
(add-to-list 'overlay-arrow-variable-list 'overlay-arrow)
(setq fringe-indicator-alist '((overlay-arrow . hollow-right-triangle)))
(overlay-put (make-overlay (pos-bol) (1+ (pos-bol))) 'before-string
             (propertize " " 'display
                         `(left-fringe breakpoint warning)))
(move-marker overlay-arrow (pos-bol))

I expect that macOS is not using overlay_p param as intended as fixed by
attached patch.  But I was having an hard time finding any documentation
proving my point other then the implementation of it's X Window System
sibling in src/xterm.c.