Mark portions of text without affecting spacing

Question

I would like to mark portions of my text that requires special attention during the review process. This marking won't be in the final document, so would like to be able to add these marks without affecting the spacing or alignment of the text. I don't want to change the color of the text, or add highlighting as that would be too distracting.

The MWE below results in additional spacing which I want to avoid. I attempted to use \kern-\widthof{} but could not get that to work.

I am not stuck on this approach so if someone has an alternative way to do this that is fine.

\documentclass{article}
\usepackage{amsmath}
\usepackage{mathabx}
\usepackage{xcolor}
\usepackage{calc}

\DeclareRobustCommand{\MarkStart}{\textcolor{green}{\raisebox{5pt}[0pt][0pt]{$\Rsh$}}}%
\DeclareRobustCommand{\MarkEnd}{\textcolor{green}{\raisebox{5pt}[0pt][0pt]{$\Lsh$}}}%

\newcommand*{\KernMarkStart}{\kern-\widthof{\MarkStart}}%
\newcommand*{\KernMarkEnd}{\kern-\widthof{\MarkEnd}}%

\newcommand*{\MarkStartWithKern}{\MarkStart\KernMarkStart}%
\newcommand*{\MarkEndWithKern}{\MarkEnd\KernMarkEnd}%

\newcommand*{\Mark}[1]{\MarkStart#1\MarkEnd}%
%\newcommand*{\Mark}[1]{\MarkStartWithKern#1\MarkEndWithKern}%
\begin{document}

Without changing the spacing, I want to mark
this word.

Without changing the spacing, I want to mark
\Mark{this} word.

\bigskip
Similary with inline math, I want to mark the $y^2$ term:

$x^2 + y^2 = z^2$.

$x^2 + \Mark{y^2} = z^2$.

\bigskip
With display mode math things should also be marked similarily:
\begin{align*}
a &= b\\
x^2 + y^2 &= z^2\\
x^2 + \Mark{y^2} &= z^2\\
c &= d
\end{align*}

and should be able to mark entire equation as:
\begin{align*}
a &= b\\
\Mark{x^2 + y^2 &= z^2}\\
c &= d
\end{align*}

The above should align with:
\begin{align*}
a &= b\\
x^2 + y^2 &= z^2\\
c &= d
\end{align*}
\end{document}

I am using the solution from Changbars to indicate location of conditional text so that the location of these markings is easy to locate so the fact that these markings don't stand out is ok.

Answer 1

\DeclareRobustCommand{\MarkStart}{{%
  \setbox0=\llap{\raisebox{5pt}{\color{green}$\Rsh$\kern-6pt}}%
  \ht0=0pt \dp0=0pt \leavevmode\box0
}}
\DeclareRobustCommand{\MarkEnd}{{%
  \setbox0=\rlap{\raisebox{5pt}{\color{green}\kern-6pt$\Lsh$}}%
  \ht0=0pt \dp0=0pt \leavevmode\box0
}}
\newcommand*{\Mark}[1]{\MarkStart#1\MarkEnd}

I set the symbols in a box that's given zero width (via \llap or \rlap), height and depth, which is then typeset. Beware that in math mode it behaves as an ordinary symbol, so it can affect spacing.