Flickering tooltips if no mouse cursor theme "loaded"

Steps to reproduce

Since 14d22cb3, GTK reads the setting gtk-cursor-theme-size when it's about to show a tooltip. This works if you're running a full GNOME session, but may not work on "lightweight" setups. For example:

Have your ~/.xinitrc contain nothing but openbox and launch an X11 session via startx.
No special themes, no extra stuff in ~/.config. (I actually used a virtual machine with a fresh and naked installation of Arch Linux.)
Run tooltip.c.
Move the mouse cursor over the new window. If your machine is slow, you can see that the tooltip flickers (appear, disappear, appear, disappear, ...). If your machine is too fast, you just see no tooltip.

A workaround is to put something like gtk-cursor-theme-size = 16 in your ~/.config/gtk-3.0/settings.ini.

Besides my example program, "real" applications show the same behavior:

The tooltips of the toolbars in LibreOffice Writer flicker.
GNOME Terminal, when showing tooltips for the recently introduced explicit hyperlinks: Run printf '\033]8;;%s\a%s\033]8;;\a' hello world at the shell, then hover over the word world.
Some people on the Manjaro forum have multiple reports.

Version information

GTK+ 3.24.1+2+ged57e564-1 (i.e., commit ed57e564 of the gtk-3-24 branch)
Arch Linux