You could do something like this (I assume that you are using pdflatex). There is imho no sensible way to get line breaks inside such todonotes, so I set a maximum size of 5cm. But as they will destroy the layout anyway I don't think that it matters much that you get large line spacing or bad line breaks with long notes.
The code naturally change all inline todos.
\documentclass{article}
\usepackage{todonotes,varwidth}
\makeatletter
\tikzstyle{diaanotestyle} = [
draw=\@todonotes@currentbordercolor,
fill=\@todonotes@currentbackgroundcolor,
line width=0.5pt,
inner sep = 0.8 ex,
rounded corners=4pt,align=left,
]
\renewcommand{\@todonotes@drawInlineNote}{%
{\begin{tikzpicture}[remember picture,baseline={(0,0)}]%
\draw node[diaanotestyle,font=\@todonotes@sizecommand,anchor=base west]{%
\begin{varwidth}[t]{5cm}
\if@todonotes@authorgiven%
{\@todonotes@sizecommand \@todonotes@author:\,\@todonotes@text}%
\else%
{\@todonotes@sizecommand \@todonotes@text}%
\fi
\end{varwidth}};%
\end{tikzpicture}}%
}%
\makeatother
\author{diaa}
\begin{document}
some text \todo[inline]{my note} another text \todo[inline]{a long comment a long comment a long comment a long commenta long comment a long comment} abllclc blblb blblb blblb blblb blblbl blblb
\end{document}
Edit
If you really need line breaks in the notes you can try to use this answer Cool Text Highlighting in LaTeX. Be aware that the soul code is rather fragile. Not everything can be put in the note text without protection. Read the documentation.
As an example (I don't have the time to adjust the style):
\documentclass{article}
\usepackage{todonotes}
\usepackage{soul}
\usepackage{tikz}
\usetikzlibrary{calc}
\usetikzlibrary{decorations.pathmorphing}
\makeatletter
\newcommand{\defhighlighter}[3][]{%
\tikzset{every highlighter/.style={color=#2, fill opacity=#3, #1}}%
}
\defhighlighter{yellow}{.5}
\newcommand{\highlight@DoHighlight}{
\fill [ decoration = {random steps, amplitude=1pt, segment length=15pt}
, outer sep = -15pt, inner sep = 0pt, decorate
, every highlighter, this highlighter ]
($(begin highlight)+(0,8pt)$) rectangle ($(end highlight)+(0,-3pt)$) ;
}
\newcommand{\highlight@BeginHighlight}{
\coordinate (begin highlight) at (0,0) ;
}
\newcommand{\highlight@EndHighlight}{
\coordinate (end highlight) at (0,0) ;
}
\newdimen\highlight@previous
\newdimen\highlight@current
\DeclareRobustCommand*\highlight[1][]{%
\tikzset{this highlighter/.style={#1}}%
\SOUL@setup
%
\def\SOUL@preamble{%
\begin{tikzpicture}[overlay, remember picture]
\highlight@BeginHighlight
\highlight@EndHighlight
\end{tikzpicture}%
}%
%
\def\SOUL@postamble{%
\begin{tikzpicture}[overlay, remember picture]
\highlight@EndHighlight
\highlight@DoHighlight
\end{tikzpicture}%
}%
%
\def\SOUL@everyhyphen{%
\discretionary{%
\SOUL@setkern\SOUL@hyphkern
\SOUL@sethyphenchar
\tikz[overlay, remember picture] \highlight@EndHighlight ;%
}{%
}{%
\SOUL@setkern\SOUL@charkern
}%
}%
%
\def\SOUL@everyexhyphen##1{%
\SOUL@setkern\SOUL@hyphkern
\hbox{##1}%
\discretionary{%
\tikz[overlay, remember picture] \highlight@EndHighlight ;%
}{%
}{%
\SOUL@setkern\SOUL@charkern
}%
}%
%
\def\SOUL@everysyllable{%
\begin{tikzpicture}[overlay, remember picture]
\path let \p0 = (begin highlight), \p1 = (0,0) in \pgfextra
\global\highlight@previous=\y0
\global\highlight@current =\y1
\endpgfextra (0,0) ;
\ifdim\highlight@current < \highlight@previous
\highlight@DoHighlight
\highlight@BeginHighlight
\fi
\end{tikzpicture}%
\the\SOUL@syllable
\tikz[overlay, remember picture] \highlight@EndHighlight ;%
}%
\SOUL@
}
\renewcommand{\@todonotes@drawInlineNote}{%
{%
\expandafter\highlight\expandafter{\@todonotes@text }%
}}%
\makeatother
\author{diaa}
\begin{document}
some text \todo[inline]{my note} another text \todo[inline]{a long comment a long comment a long comment a long commenta long comment a long comment} abllclc blblb blblb blblb blblb blblbl blblb
\end{document}
some text \colorbox{orange}{my text} another text
. If they are to be inserted between lines without affecting the underlying spacing, perhaps an answer like this, tex.stackexchange.com/questions/142242/…, may help.todonotes
features. So, it is better for me to change the way inline notes are printed.