When you have enabled `left-margin-width' and `right-margin-width'
as well as disabled the `x-gtk-use-system-tooltips', the text will
be invisible when running `x-show-tip'. The tip will be show, but
not visible.

Steps to reproduce with emacs -Q
(setq-default
  left-margin-width 1
  right-margin-width 1)

(setq x-gtk-use-system-tooltips nil)

(x-show-tip "Can you see me now?")

In GNU Emacs 25.3.1 (x86_64-pc-linux-gnu, GTK+ Version 3.22.19)
 of 2017-09-16 built on juergen
Windowing system distributor 'The X.Org Foundation', version 11.0.11905000
Configured using:
 'configure --prefix=/usr --sysconfdir=/etc --libexecdir=/usr/lib
 --localstatedir=/var --with-x-toolkit=gtk3 --with-xft --with-modules
 'CFLAGS=-march=x86-64 -mtune=generic -O2 -pipe -fstack-protector-strong
 -fno-plt' CPPFLAGS=-D_FORTIFY_SOURCE=2
 LDFLAGS=-Wl,-O1,--sort-common,--as-needed,-z,relro,-z,now'

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GCONF GSETTINGS
NOTIFY ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB
TOOLKIT_SCROLL_BARS GTK3 X11 MODULES

Important settings:
  value of $LC_ALL: en_US.UTF-8
  value of $LANG: en_US.UTF-8
  value of $XMODIFIERS: @im=ibus
  locale-coding-system: utf-8-unix

Major mode: Fundamental

Minor modes in effect:
  tooltip-mode: t
  global-eldoc-mode: t
  electric-indent-mode: t
  mouse-wheel-mode: t
  file-name-shadow-mode: t
  global-font-lock-mode: t
  auto-composition-mode: t
  auto-encryption-mode: t
  auto-compression-mode: t
  buffer-read-only: t
  line-number-mode: t
  transient-mark-mode: t

Recent messages:
‘use-package’ is already installed
For information about GNU Emacs and the GNU system, type C-h C-a.
Making completion list...
funcall-interactively: End of buffer [6 times]

Load-path shadows:
None found.

Features:
(shadow sort mail-extr emacsbug sendmail cus-edit cus-start cus-load
wid-edit mm-archive message idna dired format-spec rfc822 mml mml-sec
mailabbrev gmm-utils mailheader mm-decode mm-bodies mm-encode mail-utils
network-stream nsm starttls url-http tls gnutls mail-parse rfc2231
rfc2047 rfc2045 ietf-drums url-gw url-cache url-auth url url-proxy
url-privacy url-expand url-methods url-history url-cookie url-domsuf
url-util mailcap url-handlers url-parse auth-source cl-seq eieio
eieio-core cl-macs gnus-util time-date mm-util help-fns mail-prsvr
password-cache url-vars epg finder-inf info package epg-config seq
byte-opt gv bytecomp byte-compile cl-extra help-mode easymenu cconv
cl-loaddefs pcase cl-lib mule-util tooltip eldoc electric uniquify
ediff-hook vc-hooks lisp-float-type mwheel x-win term/common-win x-dnd
tool-bar dnd fontset image regexp-opt fringe tabulated-list newcomment
elisp-mode lisp-mode prog-mode register page menu-bar rfn-eshadow timer
select scroll-bar mouse jit-lock font-lock syntax facemenu font-core
frame cl-generic cham georgian utf-8-lang misc-lang vietnamese tibetan
thai tai-viet lao korean japanese eucjp-ms cp51932 hebrew greek romanian
slovak czech european ethiopic indian cyrillic chinese charscript
case-table epa-hook jka-cmpr-hook help simple abbrev minibuffer
cl-preloaded nadvice loaddefs button faces cus-face macroexp files
text-properties overlay sha1 md5 base64 format env code-pages mule
custom widget hashtable-print-readable backquote dbusbind inotify
dynamic-setting system-font-setting font-render-setting move-toolbar gtk
x-toolkit x multi-tty make-network-process emacs)

Memory information:
((conses 16 207883 48038)
 (symbols 48 28074 0)
 (miscs 40 801 226)
 (strings 32 51742 37972)
 (string-bytes 1 1373182)
 (vectors 16 27579)
 (vector-slots 8 1318855 206622)
 (floats 8 250 210)
 (intervals 56 267 0)
 (buffers 976 60))