unicode combining characters rendering issues

[ntessore]

Describe the bug

At some point during the last couple of updates, an issue appeared with rendering unicode combining characters, in particular diacritics/accents. Take the combination x⃗ [U+0078 U+20d7] which now renders as in screenshot 1 (note the x is in column 1). Turning off the Core Text Renderer restores the correct rendering as in screenshot 2.

Edit: It works fine in snapshot 166.

To Reproduce
Copy x⃗ from here and paste it into MacVim.

Expected behavior
Render x⃗.

Screenshots

MacVim default:

Core Text Renderer disabled (or using snapshot 166):

Environment (please complete the following information):
macOS 11

$ mvim --version
VIM - Vi IMproved 8.2 (2019 Dec 12, compiled Dec 19 2020 21:23:05)
macOS version
Included patches: 1-2164
Compiled by root@Mac-1608412769109.local
Huge version with MacVim GUI.  Features included (+) or not (-):
+acl               +file_in_path      -mouse_sysmouse    -tag_old_static
+arabic            +find_in_path      +mouse_urxvt       -tag_any_white
+autocmd           +float             +mouse_xterm       -tcl
+autochdir         +folding           +multi_byte        +termguicolors
-autoservername    -footer            +multi_lang        +terminal
+balloon_eval      +fork()            -mzscheme          +terminfo
+balloon_eval_term +fullscreen        +netbeans_intg     +termresponse
+browse            +gettext           +num64             +textobjects
++builtin_terms    -hangul_input      +odbeditor         +textprop
+byte_offset       +iconv             +packages          +timers
+channel           +insert_expand     +path_extra        +title
+cindent           +ipv6              +perl/dyn          +toolbar
+clientserver      +job               +persistent_undo   +transparency
+clipboard         +jumplist          +popupwin          +user_commands
+cmdline_compl     +keymap            +postscript        +vartabs
+cmdline_hist      +lambda            +printer           +vertsplit
+cmdline_info      +langmap           +profile           +virtualedit
+comments          +libcall           +python/dyn        +visual
+conceal           +linebreak         +python3/dyn       +visualextra
+cryptv            +lispindent        +quickfix          +viminfo
+cscope            +listcmds          +reltime           +vreplace
+cursorbind        +localmap          +rightleft         +wildignore
+cursorshape       +lua/dyn           +ruby/dyn          +wildmenu
+dialog_con_gui    +menu              +scrollbind        +windows
+diff              +mksession         +signs             +writebackup
+digraphs          +modify_fname      +smartindent       -X11
+dnd               +mouse             -sound             -xfontset
-ebcdic            +mouseshape        +spell             +xim
+emacs_tags        +mouse_dec         +startuptime       -xpm
+eval              -mouse_gpm         +statusline        -xsmp
+ex_extra          -mouse_jsbterm     -sun_workshop      -xterm_clipboard
+extra_search      +mouse_netterm     +syntax            -xterm_save
-farsi             +mouse_sgr         +tag_binary        
   system vimrc file: "$VIM/vimrc"
     user vimrc file: "$HOME/.vimrc"
 2nd user vimrc file: "~/.vim/vimrc"
      user exrc file: "$HOME/.exrc"
  system gvimrc file: "$VIM/gvimrc"
    user gvimrc file: "$HOME/.gvimrc"
2nd user gvimrc file: "~/.vim/gvimrc"
       defaults file: "$VIMRUNTIME/defaults.vim"
    system menu file: "$VIMRUNTIME/menu.vim"
  fall-back for $VIM: "/Applications/MacVim.app/Contents/Resources/vim"
Compilation: clang -c -I. -Iproto -DHAVE_CONFIG_H -DFEAT_GUI_MACVIM -Wall -Wno-unknown-pragmas -pipe  -DMACOS_X -DMACOS_X_DARWIN  -g -O2 -arch x86_64 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1 -Wall -Wextra -Wshadow -Werror -Wno-error=missing-field-initializers -Wno-error=deprecated-declarations -Wno-error=unused-function       
Linking: clang   -L. -fstack-protector-strong -L/usr/local/lib -L/usr/local/opt/libyaml/lib -L/usr/local/opt/openssl@1.1/lib -L/usr/local/opt/readline/lib -L. -fstack-protector-strong -L/usr/local/lib -L/usr/local/opt/libyaml/lib -L/usr/local/opt/openssl@1.1/lib -L/usr/local/opt/readline/lib  -arch x86_64 -L/usr/local/lib -o Vim        -lm  -lncurses -liconv /usr/local/lib/libintl.a -framework AppKit   -fstack-protector  -L/System/Library/Perl/5.18/darwin-thread-multi-2level/CORE 

Additional context
Use a font that can render the glyph.
See below.

[ychin]

I can reproduce this but need to look into what a valid fix is.

[ntessore]

Is it possible that this only occurs when the selected guifont does not support the combining character, and a fallback is used?

Compare Menlo which falls back to a different x with the vector arrow:

And JuliaMono which has support for the vector arrow:

[ntessore]

@ichizok Could this be related to #680?