I should probably include this, since it seems to be a GUI font rendering issue:
VIM - Vi IMproved 7.3 (2010 Aug 15, compiled Jul 13 2012 00:11:38)
Included patches: 1-600
Compiled by ben
Huge version with GTK2 GUI. Features included (+) or not (-):
+arabic +autocmd +balloon_eval +browse ++builtin_terms +byte_offset +cindent +clientserver +clipboard +cmdline_compl
+cmdline_hist +cmdline_info +comments +conceal +cryptv +cscope +cursorbind +cursorshape +dialog_con_gui +diff +digraphs
+dnd -ebcdic +emacs_tags +eval +ex_extra +extra_search +farsi +file_in_path +find_in_path +float +folding -footer +fork()
+gettext -hangul_input +iconv +insert_expand +jumplist +keymap +langmap +libcall +linebreak +lispindent +listcmds +localmap
-lua +menu +mksession +modify_fname +mouse +mouseshape +mouse_dec +mouse_gpm -mouse_jsbterm +mouse_netterm -mouse_sysmouse
+mouse_xterm +mouse_urxvt +multi_byte +multi_lang -mzscheme +netbeans_intg +path_extra +perl/dyn +persistent_undo
+postscript +printer +profile +python/dyn -python3 +quickfix +reltime +rightleft -ruby +scrollbind +signs +smartindent
-sniff +startuptime +statusline -sun_workshop +syntax +tag_binary +tag_old_static -tag_any_white -tcl +terminfo
+termresponse +textobjects +title +toolbar +user_commands +vertsplit +virtualedit +visual +visualextra +viminfo +vreplace
+wildignore +wildmenu +windows +writebackup +X11 -xfontset +xim +xsmp_interact +xterm_clipboard -xterm_save
system vimrc file: "$VIM/vimrc"
user vimrc file: "$HOME/.vimrc"
user exrc file: "$HOME/.exrc"
system gvimrc file: "$VIM/gvimrc"
user gvimrc file: "$HOME/.gvimrc"
system menu file: "$VIMRUNTIME/menu.vim"
fall-back for $VIM: "/usr/local/share/vim"
Compilation: gcc -c -I. -Iproto -DHAVE_CONFIG_H -DFEAT_GUI_GTK -pthread -I/usr/include/gtk-2.0 -I/usr/lib/i386-linux-gnu/gtk-2.0/include -I/usr/include/atk-1.0 -I/usr/include/cairo -I/usr/include/gdk-pixbuf-2.0 -I/usr/include/pango-1.0 -I/usr/include/gio-unix-2.0/ -I/usr/include/glib-2.0 -I/usr/lib/i386-linux-gnu/glib-2.0/include -I/usr/include/pixman-1 -I/usr/include/freetype2 -I/usr/include/libpng12 -I/usr/local/include -g -O2 -U_FORTIFY_SOURCE -D_FORTIFY_SOURCE=1
Linking: gcc -Wl,-E -L/usr/local/lib -Wl,--as-needed -o vim -lgtk-x11-2.0 -lgdk-x11-2.0 -latk-1.0 -lgio-2.0 -lpangoft2-1.0 -lpangocairo-1.0 -lgdk_pixbuf-2.0 -lcairo -lpango-1.0 -lfreetype -lfontconfig -lgobject-2.0 -lglib-2.0 -lSM -lICE -lXpm -lXt -lX11 -lXdmcp -lSM -lICE -lm -ltinfo -lnsl -lselinux -lacl -lattr -lgpm -ldl -Wl,-E -fstack-protector -L/usr/local/lib -L/usr/lib/perl/5.14/CORE -lperl -ldl -lm -lpthread -lcrypt
I finally got a chance to try applying the patch and it seems to work as advertised. I have no idea whether it has unintended side effects. I'll run with it for a while to see if I notice any. But I don't usually use modeless selection for anything so I doubt I'd see anything in my normal use.