Upon more investigation, neither \fontdimen60 nor \fontdimen59 is used
by XeTeX as RadicalVGap when constructing radicals (testing code
available at
https://github.com/latex3/unicode-math/issues/671#issuecomment-3815161944).
It's reasonable to suspect that neither \fontdimen61
(RadicalRuleThickness) nor \fontdimen62 (RadicalExtraAscender) is
used.
And for Overbar and Underbar, \fontdimen's 53 through 58 are all
ignored by XeTeX. For testing, TeX Gyre DejaVu Math is a great font,
since it has an inconsistent \fontdimen48 (FractionRuleThickness).
Here's the testing code for Overbar and Underbar:
% plain XeTeX
\def\tmp#1{\textfont#1\hundred \scriptfont#1\seventy
\scriptscriptfont#1\fifty}
\def\setupfont#1{\font\hundred="[#1.otf]:script=math" at 100.33333pt
\font\seventy="[#1.otf]:script=math;+ssty=0" at 70.33333pt
\font\fifty="[#1.otf]:script=math;+ssty=1" at 50.33333pt
\tmp0\tmp1\tmp2\tmp3%
\baselineskip12pt\noindent--- #1 ---\par
\immediate\write-1{--- #1 ---}}
\Umathcode`\.=0 0 `\.
\def\choosefont#1{\ifx#1\displaystyle \textfont
\else \ifx#1\textstyle \textfont
\else \ifx#1\scriptstyle \scriptfont
\else \scriptscriptfont \fi \fi \fi}
\def\maath{\mathsurround=0pt }
\def\red{\special{color push rgb 1 0 0}\hrule\special{color pop}}
\def\testoverbar#1{% #1: one of four math styles
% follow the OpenType MATH spec:
\setbox0\hbox{\vbox{\red
\kern \fontdimen55 \choosefont#12
\hrule height \fontdimen54 \choosefont#12
\kern \fontdimen53 \choosefont#12
\hbox{$#1.\maath$}}}%
% what XeTeX did:
\setbox1\hbox{\vbox{\red\hbox{$#1\overline.\maath$}}}%
% emulate the wrong construction:
\setbox2\hbox{\vbox{\red
\kern \fontdimen48 \choosefont#12
\hrule height \fontdimen48 \choosefont#12
\kern 3\fontdimen48 \choosefont#12
\hbox{$#1.\maath$}}}%
\immediate\write-1{overbar emulation \string#1:
ht diff \the\dimexpr\ht2-\ht1.}%
\baselineskip\glueexpr\fontdimen6 \choosefont#12*7/10\relax
\hbox{\unhbox0\ \unhbox1\ \unhbox2\ overbar \tt\string#1}%
}
\def\testunderbar#1{% #1: one of four math styles
% follow the OpenType MATH spec:
\setbox0\hbox{\vtop{\hbox{$#1.\maath$}%
\kern \fontdimen56 \choosefont#12
\hrule height \fontdimen57 \choosefont#12
\kern \fontdimen58 \choosefont#12 \red}}%
% what XeTeX did:
\setbox1\hbox{\vtop{\hbox{$#1\underline.\maath$}\red}}%
% emulate the wrong construction:
\setbox2\hbox{\vtop{\hbox{$#1.\maath$}%
\kern 3\fontdimen48 \choosefont#12
\hrule height \fontdimen48 \choosefont#12
\kern \fontdimen48 \choosefont#12 \red}}%
\immediate\write-1{underbar emulation \string#1:
dp diff \the\dimexpr\dp2-\dp1.}%
\baselineskip\glueexpr\fontdimen6 \choosefont#12*7/10\relax
\hbox{\unhbox0\ \unhbox1\ \unhbox2\ underbar \tt\string#1}%
}
\raggedbottom
\setupfont{texgyredejavu-math}
\testoverbar\displaystyle \testoverbar\textstyle
\testoverbar\scriptstyle \testoverbar\scriptscriptstyle
\testunderbar\displaystyle \testunderbar\textstyle
\testunderbar\scriptstyle \testunderbar\scriptscriptstyle
\bye
