sim: display flickers

since my last git pull i see a flicker on the display. i biseced the behaviour and found

d73ac07d7f10093638e82508fb7fd352ef5df93e is the first bad commit
commit d73ac07d7f10093638e82508fb7fd352ef5df93e
Author: Øyvind Kolås <pippin@gimp.org>
Date:   Sat Sep 2 03:06:31 2023 +0200

    st3m_gfx: add a mutex around access to overlay

it is the change in line 137 in python_payload/st3m/ui/elements/overlays.py:

    sys_display.update(octx)

if i comment the line everything looks fine again.