Non-CoreText renderer handles line height wrong with font fallback glyphs

When using the non-CoreText renderer, when font fallback is used to draw a glyph (because the active font doesn't have it), MacVim doesn't force the line height of the fallback glyph to match the original line height. So when the fallback font has a larger line height, it actually increases the line height for the whole line, which causes the whole line to render a bit too low.

Here's a screenshot of it in action. Two lines in this screenshot are using concealed characters that display ⟶ (U+27F6 LONG RIGHTWARDS ARROW), and those lines draw wrong. Even the line number is wrong.

My 'guifont' is set to Menlo:h11. I'm pretty sure it's falling back to Code2000 in this screenshot, which is a third-party font that I have installed. The two system fonts that include ⟶ are Apple Symbols and STIXGeneral. If you try to reproduce this without Code2000 and it uses Apple Symbols as fallback, it'll probably look fine since Apple Symbols actually has a shorter line height than Menlo does. STIXGeneral should reproduce it, but I don't know which font has higher precedence for fallback. But it's definitely very obvious with Code2000.

OS X 10.11.3 (15D21), MacVim 7.4-96 (via Homebrew).

> vim --version
VIM - Vi IMproved 7.4 (2013 Aug 10, compiled Feb 14 2016 15:39:36)
MacOS X (unix) version
Included patches: 1-1265
Compiled by Homebrew
Huge version with MacVim GUI.  Features included (+) or not (-):
+acl             +farsi           +mouse_netterm   +syntax
+arabic          +file_in_path    +mouse_sgr       +tag_binary
+autocmd         +find_in_path    -mouse_sysmouse  +tag_old_static
+balloon_eval    +float           +mouse_urxvt     -tag_any_white
+browse          +folding         +mouse_xterm     -tcl
++builtin_terms  -footer          +multi_byte      +terminfo
+byte_offset     +fork()          +multi_lang      +termresponse
+channel         +fullscreen      -mzscheme        +textobjects
+cindent         -gettext         +netbeans_intg   +title
+clientserver    -hangul_input    +odbeditor       +toolbar
+clipboard       +iconv           +path_extra      +transparency
+cmdline_compl   +insert_expand   +perl            +user_commands
+cmdline_hist    +jumplist        +persistent_undo +vertsplit
+cmdline_info    +keymap          +postscript      +virtualedit
+comments        +langmap         +printer         +visual
+conceal         +libcall         +profile         +visualextra
+cryptv          +linebreak       +python          +viminfo
+cscope          +lispindent      -python3         +vreplace
+cursorbind      +listcmds        +quickfix        +wildignore
+cursorshape     +localmap        +reltime         +wildmenu
+dialog_con_gui  +lua             +rightleft       +windows
+diff            +menu            +ruby            +writebackup
+digraphs        +mksession       +scrollbind      -X11
+dnd             +modify_fname    +signs           -xfontset
-ebcdic          +mouse           +smartindent     +xim
+emacs_tags      +mouseshape      -sniff           -xsmp
+eval            +mouse_dec       +startuptime     -xterm_clipboard
+ex_extra        -mouse_gpm       +statusline      -xterm_save
+extra_search    -mouse_jsbterm   -sun_workshop    -xpm
   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: "$HOME/.gvimrc"
2nd user gvimrc file: "~/.vim/gvimrc"
    system menu file: "$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_UNIX  -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -I/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/System/Library/Frameworks/Python.framework/Versions/2.7/include/python2.7 -fno-strict-aliasing -fno-common -dynamic -arch x86_64 -arch i386 -g -Os -pipe -fno-common -fno-strict-aliasing -fwrapv -DENABLE_DTRACE -DMACOSX -DNDEBUG -Wall -Wstrict-prototypes -Wshorten-64-to-32 -DNDEBUG -g -fwrapv -Os -Wall -Wstrict-prototypes -DENABLE_DTRACE -isysroot /Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk -mmacosx-version-min=10.11 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1      
Linking: clang   -L. -L/usr/local/lib -L. -L/usr/local/lib -L/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/config -lpython2.7 -framework CoreFoundation -Wl,-syslibroot,/Applications/Xcode.app/Contents/Developer/Platforms/MacOSX.platform/Developer/SDKs/MacOSX10.11.sdk -mmacosx-version-min=10.11 -L/usr/local/lib -o Vim -framework Cocoa -framework Carbon       -lm  -lncurses -liconv -framework Cocoa  -L/usr/local/lib -llua -fstack-protector  -L/System/Library/Perl/5.18/darwin-thread-multi-2level/CORE -lperl -framework Python   -framework Ruby    

Reproduce el problema en el renderizador que no es CoreText con Menlo:h11, una fuente alternativa como Code2000 o STIXGeneral y el glifo U+27F6. Empieza rastreando cómo el renderizador que no es CoreText gestiona la altura de línea para los glifos de fuentes alternativas. Se considera terminado cuando los glifos de fuentes alternativas ya no aumenten la altura de línea ni desplacen verticalmente el texto y los números de línea.