The BIST for "tightinset" intermittently fails on the buildbots, e.g.:
http://buildbot.octave.org:8010/#/builders/35/builds/86/steps/7/logs/stdio

Relevant part from the log:

>>>>> processing /scratch/buildbot/workers/jwe-debian-x86_64-2/stable-gcc-lto-debian/build/libinterp/corefcn/graphics.cc-tst
***** testif HAVE_OPENGL, HAVE_QT; have_window_system () && any (strcmp ("qt", available_graphics_toolkits ()))
 hf = figure ("visible", "off");
 graphics_toolkit (hf, "qt");
 fpos = get (hf, "position");
 unwind_protect
   plot (rand (3));
   position = get (gca, "position");
   outerposition = get (gca, "outerposition");
   looseinset = get (gca, "looseinset");
   tightinset = get (gca, "tightinset");
   set (hf, "position", [fpos(1:2), 2*fpos(3:4)]);
   set (hf, "position", fpos);
   assert (get (gca, "outerposition"), outerposition, 0.001);
   assert (get (gca, "position"), position, 0.001);
   assert (get (gca, "looseinset"), looseinset, 0.001);
   assert (get (gca, "tightinset"), tightinset, 0.001);
 unwind_protect_cleanup
   close (hf);
 end_unwind_protect
!!!!! test failed
ASSERT errors for:  assert (get (gca, "tightinset"),tightinset,0.001)
  Location  |  Observed  |  Expected  |  Reason
    (1)        0.055357     0.044643     Abs err 0.010714 exceeds tol 0.001 by 0.01

This might be caused by a concurrency issue between the interpreter and GUI threads.

I haven't found a dedicated bug report.