Is it what you want ?
\documentclass{article}
\usepackage{xcolor}
\usepackage{colortbl}
\usepackage{booktabs}
\usepackage{adjustbox}
\usepackage{tabu}
\begin{document}
\belowrulesep=0pt
\aboverulesep=0pt
\begin{center}
\begin{tabu}{X>{\columncolor{blue}}XXX}
1 & 2 & a & b\\
1 & 2 & a & b\\
\tabucline[green]-
1 & 2 & a & b\\
\arrayrulecolor{red}
\cmidrule(r{2.0ex}){1-2}\cmidrule(l{2.0ex}){3-4}
\adjustbox{margin=0pt 10pt 0pt 0pt}{1} & 2 & \cellcolor{green}a & b\\
\tabucline[blue]-
\adjustbox{margin=0pt 0pt 0pt 10pt}{1} & 2 & \cellcolor{green}a & b\\
\end{tabu}
\end{center}
\end{document}
This is more a hack than a real solution. I add margin inside the cells with adjustbox, and remove the space around cmidrule ....
Here's my version:
\documentclass{article}
\usepackage{mdframed,lipsum}
\parindent=0pt
\newmdenv[leftmargin=\dimexpr-0.5em-3pt, innerleftmargin=0.5em,
rightmargin=\dimexpr-0.5em-3pt, innerrightmargin=0.5em,
linewidth=3pt,linecolor=red, topline=false, bottomline=false,
innertopmargin=0pt,innerbottommargin=0pt,skipbelow=0pt,skipabove=0pt,
]{notex}
\newenvironment{note}
{\par\vskip\dimexpr\dp\strutbox-\prevdepth\relax\notex\strut\ignorespaces}
{\par\xdef\notetpd{\the\prevdepth}\endnotex\vskip-\notetpd\relax}
\begin{document}
\leavevmode\llap{\smash{\vrule depth7\baselineskip height0pt\hskip1em}}%
\lipsum*[2]
\begin{note}
\lipsum[2]
\end{note}
\leavevmode\llap{\smash{\vrule height\baselineskip\hskip1em}}%
\lipsum[2]
\end{document}
The black rules are just to show that the alignment is correct (they can't be too near the text because mdframed applies a white background).
A magnified view of the top line
And one of the bottom line
At the start we ensure to skip by the glue TeX would insert and we add a strut to the first line in the note environment, so to have the correct distance between baselines. At the end we do similarly, keeping into account the depth of the last line in the note environment.
Brief explanation
mdframed sets its chunks in a \vbox which is inserted in the main vertical list; this usually upsets the interline spacing, as a \vbox has its reference point at the baseline of the last box inside it. The package tries to do clever things not to add unwanted spaces, but in this case it's not sufficient.
So we need to emulate TeX's normal behavior. The distance from one baseline and the next is the sum of \dp\strutbox and \ht\strutbox; we can insert a strut in the first line of note which makes it the correct height; but adding \dp\strutbox vertical space will usually be too much, because the last line before note may have descenders. So we end the paragraph so that \prevdepth (an internal parameter) contains the depth of the last contributed box (precisely the last line). So we can add
\vskip\dp\strutbox-\prevdepth
and all is good. Something similar we have to do at the end. We end the paragraph and define \notetpd to contain the value of \prevdepth, so we can remove a vertical space with that amount, putting everything back in synch.
Best Answer
I looked at the definition of
\evin thephysicspackage. In your case it essentially reduces toThe
\vphantom{\hat{O}_P}will insert a box of zero width and of type mathord, but when followed by a mathopen atom (which\leftmakes automatically), spacing is inserted.The definition of
\evmakes absolutely no sense. The implementers are trying to reverse the effect of the automatic scaling of\leftand\rightby using\smashto hide the height of the content, but then the construct will have no height and depth, so they reinserted using\vphantom.The package code is also full of spurious spaces. Considering the quality of the implementation, the best way to fix this issue is by not using the
physicspackage.