file-icons font causing issues with one glyph

[bhrgunatha]

Arch Linux - fully up-to-date
Emacs 29.0.50 (but I get the same in 28.1)
all-the-icons - 20220427.1911 (from melpa)

This is a weird one and has taken me a long time to discover the culprit.

When I visit Get Emoji all emojis are displayed using my system installed emoji font JoyPixels - 6.6.

Except for one.

U+1F441 - 👁 is rendered from the file-icons.ttf font installed by all-the-icons at $HOME/.local/share/fonts/

This happens in Firefox 100 and Chromium 101.

Partial page Screenshot - Firefox
Partial page Screenshot - Chromium

LibreOffice apps Writer and Calc both display correctly using the system emoji font.
Copy/paste that bottom line of Gestures and Body Parts emoji from GetEmoji.com into emacs:
Emacs partial screenshot
U+1F441 isn't even getting rendered in emacs.

If I move file-icons.ttf somewhere else, the JoyPixels font is used (as I think it always should be).

mv $HOME/.local/share/fonts/file-icons.ttf $HOME/stash  

Here are the results:

• Firefox
• Chromium
• Emacs is still unhappy though

I'm scratching my head over this.

Why would one single glyph cause these problems?
More importantly how can we fix it so file-icons doesn't stomp over the system emoji font.

[bhrgunatha]

After some more digging:

I can insert the correct character in emacs (29 ftw!)
C-x 8 e i
b - body
b - body parts
o - the 👁️ emoji

Next to that I copy/paste the weird character from the web page - 👁

Highlight the correct emoji

M-x describe-char

             position: 1 of 6 (0%), column: 0
            character: 👁 (displayed as 👁) (codepoint 128065, #o372101, #x1f441)
              charset: unicode (Unicode (ISO10646))
code point in charset: 0x1F441
               script: symbol
               syntax: w 	which means: word
             category: .:Base
             to input: type "C-x 8 RET 1f441" or "C-x 8 RET EYE"
          buffer code: #xF0 #x9F #x91 #x81
            file code: #xF0 #x9F #x91 #x81 (encoded by coding system utf-8-unix)
              display: composed to form "👁️" (see below)

Composed with the following character(s) "️" using this font:
  ftcrhb:-GOOG-JoyPixels-regular-normal-normal-*-21-*-*-*-*-0-iso10646-1
by these glyphs:
  [0 1 128065 557 25 0 25 20 6 nil]
with these character(s):
  ️ (#xfe0f) VARIATION SELECTOR-16

Character code properties: customize what to show
  name: EYE
  general-category: So (Symbol, Other)
  decomposition: (128065) ('👁')

There are 2 overlays here:
 From 1 to 5
  face                 hl-line
  priority             -50
  window               #<window 25 on *text*>
 From 1 to 3
  face                 region
  priority             (nil . 100)
  window               #<window 25 on *text*>

[back]

highlight the bad character - M-x describe-char

             position: 3 of 6 (33%), column: 1
            character: 👁 (displayed as 👁) (codepoint 128065, #o372101, #x1f441)
              charset: unicode (Unicode (ISO10646))
code point in charset: 0x1F441
               script: symbol
               syntax: w 	which means: word
             category: .:Base
             to input: type "C-x 8 RET 1f441" or "C-x 8 RET EYE"
          buffer code: #xF0 #x9F #x91 #x81
            file code: #xF0 #x9F #x91 #x81 (encoded by coding system utf-8-unix)
              display: by this font (glyph code):
    x:-monotype-arial unicode ms-medium-r-normal--21-152-100-100-p-208-gb18030.2000-0 (#x9439CF39)

Character code properties: customize what to show
  name: EYE
  general-category: So (Symbol, Other)
  decomposition: (128065) ('👁')

There are 2 overlays here:
 From 1 to 7
  face                 hl-line
  priority             -50
  window               #<window 25 on *text*>
 From 3 to 4
  face                 region
  priority             (nil . 100)
  window               #<window 25 on *text*>

[back]

I've no idea what is causing the composition (when the correct glyph is displayed) nor why file-icons.ttf triggers this strange behaviour.