Help with unicode diacritics

[Stephen Eglen <sje30@cam.ac.uk>]

Hello,

can anyone out here help me with debugging a Unicode font issue? In
particular, I'd like to use the COMBINING OVERLINE (x0305) code to make
"x" with a bar above it (i.e. x bar, to represent the mean of x).

I'm on manjaro arch, with Emacs 27.1.  I wrote the following snippet to
demonstrate the problem. See the line of a character a with various
diacritics under the call to (code) to demonstrate the problem.  Out of
the three fonts I tested, Monaco does the best, but ideally I'd like to
use JuliaMono.  If I view this same file in xfce4-terminal, using
JuliaMono, all the diacritics appear fine.  This makes me suspect Emacs.
On the page: https://damtp.cam.ac.uk/user/sje30/temp/accents.png is what
I see (using EXWM, with emacs rendering on left and xfce-terminal on the
right).

How might I debug further?

The JuliaMono font was installed using the arch package
https://github.com/cormullion/juliamono


Alternatively, does anyone recommend a monospace font with good Unicode
performance in Emacs?

Thanks for any pointers, Stephen


(defun code ()
  "Insert a range of unicode diactrics into buffer."
  (dotimes (i 20)
    (insert "a")
    (insert (+ 769 i))
    (insert " ")
  ))

;; x0301 is code point  (+ (* 3 256) 1) => 769
;; (code)
;; á â ã ā a̅ ă ȧ ä ả å a̋ ǎ a̍ a̎ ȁ a̐ ȃ a̒ a̓ a̔ 

;; Monaco renders ok except a̐
;;  (set-face-attribute 'default nil :font "Monaco")
;;
;; quite a few missing 
;; (set-face-attribute 'default nil :font "JuliaMono")
;;
;; quite a few missing
;; (set-face-attribute 'default nil :font "Roboto Mono")






(emacs-version)
"GNU Emacs 27.1 (build 1, x86_64-pc-linux-gnu, GTK+ Version 3.24.22, cairo version 1.17.3)
 of 2020-08-28"