bug#30399: 27.0.50; tooltips are broken

[Stephen Berman <stephen.berman@gmx.net>]

I'm seeing buggy behavior with tooltips on master, and though I haven't
bisected with git, I see the problems starting with my first build after
this commit:

e462308f03..: Martin Rudalics 2018-01-18 Fix some tooltip related problems

and I don't see the problems in my builds before that commit.  My builds
are with GTK+; I haven't tried with another or no toolkit.

Here's the (expected) behavior prior to that commit (also seen in
current emacs-26):

0. emacs -Q
1. evaluate (tooltip-show "This is a test")
   => A GTK+-themed tooltip is displayed for 10 seconds or until there
   is an input event, then disappears. 
2. evaluate (x-show-tip "This is a test")
   => A GTK+-themed tooltip is displayed for 5 seconds or until there
   is an input event, then disappears.
3. evaluate (let (x-gtk-use-system-tooltips)
              (tooltip-show "This is a test"))
   => A non-toolkit tooltip is displayed for 10 seconds or until there
   is an input event, then disappears.
4. evaluate (let (x-gtk-use-system-tooltips)
              (x-show-tip "This is a test"))
   => A non-toolkit tooltip is displayed for 5 seconds or until there
   is an input event, then disappears.

And here's the buggy behavior (starting with step 2) I see on master
since the above commit:

0. emacs -Q
1. evaluate (tooltip-show "This is a test")
   => A GTK+-themed tooltip is displayed for 10 seconds or until there
   is an input event, then disappears.
2. evaluate (x-show-tip "This is a test")
   => A GTK+-themed tooltip is displayed and remains displayed, even if
   there are input events, until executing step 3 or repeating step 1.
3. evaluate (let (x-gtk-use-system-tooltips)
              (tooltip-show "This is a test"))
   => A non-toolkit tooltip is displayed and remains displayed, even if
   there are input events, until the end of the Emacs session (at least
   I haven't found a way to get rid of it); however, if the GTK+-themed
   tooltip from step 2 is still displayed when the above sexp is
   evaluated, then after 10 (not 5) seconds the GTK+-themed tooltip
   disappears (but the non-toolkit tooltip remains).
4. evaluate (let (x-gtk-use-system-tooltips)
              (x-show-tip "This is a test"))
   => A non-toolkit tooltip is displayed and remains displayed, even if
   there are input events, until the end of the Emacs session AFAICT; if
   the tooltip from step 3 is still displayed when the above sexp is
   evaluated, it is just moved by this step but does not disappear, and
   if the GTK+-themed tooltip from step 2 is still displayed that
   tooltip also remains displayed (unlike in step 3).

I tried stepping through Fx_show_tip in gdb while executing step 1 and
again with step 2 in current master.  In both cases the GTK+-themed
tooltip appears, the code path jumps to start_timer, runs a timer, calls
x-hide-tip and then the function returns; in step 1, on continuing, the
tooltip disappears, but in step 2, on continuing, the tooltip remains.
Then I tried stepping through x_hide_tip, but the breakpoint was hit
before the tooltip was displayed and on continuing until the command
loop was again accessible, any input immediate hit the breakpoint
again.  So I probably cannot debug this further without guidance.

In GNU Emacs 27.0.50 (build 17, x86_64-pc-linux-gnu, GTK+ Version 3.22.18)
 of 2018-02-08 built on rosalinde
Repository revision: a710f8aa61ca73054109dc4f926d1ac6aabdd849
Windowing system distributor 'The X.Org Foundation', version 11.0.11903000
System Description: Linux From Scratch

Configured using:
 'configure --with-xwidgets 'CFLAGS=-Og -g3''

Configured features:
XPM JPEG TIFF GIF PNG RSVG IMAGEMAGICK SOUND GPM DBUS GSETTINGS NOTIFY
ACL GNUTLS LIBXML2 FREETYPE M17N_FLT LIBOTF XFT ZLIB TOOLKIT_SCROLL_BARS
GTK3 X11 THREADS XWIDGETS LCMS2

Important settings:
  value of $LANG: en_US.UTF-8
  locale-coding-system: utf-8-unix