To answer that specific question (what's the problem), I believe
    it's the X11 default "modesetting" driver.

    Try running 'lsmod' and see if you see 'modesetting' in the report;
    if so, that's likely the cause. Or grep your Xorg.0.log for "modesetting".

    Pretty sure the presence and active use of that driver is the cause.

    If you install the actual manufacturer graphics drivers, that 'modesetting'
    default driver should go away, and with it, the problem should go too.

    For a workaround in FLTK, see the other replies in this thread which
    would set the linewidth to 1 when drawing the focus box.

    The github issue Albrecht mentioned refers back to an older, long thread
    that goes into the details of using linewidth = 1 and speed issues it can cause
    if used across the board, which is why we continue to use linewidth=0 as the default:
    https://groups.google.com/g/fltkcoredev/c/K8Pc0O-dCs0/m/CZ5rMRhmCQAJ

    But I'd agree we could/should modify just the focus box drawing code
    to set the linewidth to 1, just for portability, as I think Manolo had recommended
    in the github thread. I wouldn't expect any significant slowdown from just focus box drawing.