Threeparttable align footnotes containing brackets

Question

I use threeparttable to format my tables. I also use brackets for footnote references. As you can see in the following picture, the 2nd line of footnote [a] is not aligned. The line starting with 0.0 is starting to far left.

How can I correct this?

Best regards, wewa00

PS: Here is the sample LaTeX code to reproduce this issue.

% Preview source code

%% LyX 2.3.6 created this file.  For more info, see http://www.lyx.org/.
%% Do not edit unless you really know what you are doing.
\documentclass[12pt,english]{article}
\renewcommand{\familydefault}{\sfdefault}
\usepackage[T1]{fontenc}
\usepackage[latin9]{inputenc}
\usepackage{geometry}
\geometry{verbose,tmargin=2.5cm,bmargin=2.5cm,lmargin=3cm,rmargin=2.5cm}
\setlength{\parskip}{\medskipamount}
\setlength{\parindent}{0pt}
\usepackage{array}
\usepackage{setspace}
\onehalfspacing

\makeatletter

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% LyX specific LaTeX commands.
%% Because html converters don't know tabularnewline
\providecommand{\tabularnewline}{\\}

%%%%%%%%%%%%%%%%%%%%%%%%%%%%%% User specified LaTeX commands.
% --------- change figure and table caption size -----------
\usepackage{caption} % to change font size of image caption
\captionsetup[figure]{font=footnotesize,labelfont=footnotesize} % available values are scriptsize, footnotesize, small, normalsize, large, and Large.
\captionsetup[table]{font=footnotesize,labelfont=footnotesize} % available values are scriptsize, footnotesize, small, normalsize, large, and Large.
% -----------------------------------------------
\usepackage{threeparttable}
\usepackage{booktabs} % booktabs: to support centering of table
\usepackage{etoolbox} % to support footnotesize within table
\appto\TPTnoteSettings{\footnotesize} % footnote font size within table
\def\tnote#1{\protect\TPToverlap{\textsuperscript{[\TPTtagStyle{#1}]}}} % brackets in table footnotes

\makeatother

\usepackage{babel}
\begin{document}
\begin{table}
\caption{}

\centering
\begin{threeparttable}

\begin{tabular}{>{\centering}m{1.2cm}>{\centering}m{4cm}>{\centering}m{3cm}>{\centering}m{1.8cm}>{\centering}m{1.9cm}}
\hline 
\noalign{\vskip\doublerulesep}
1 & 2 & 3 & 4 & 5\tabularnewline[\doublerulesep]
\hline 
\noalign{\vskip\doublerulesep}
\end{tabular}

\begin{tablenotes}
\item [a] Loremips umdolor sit ahmet 0.00 lore mipsum, 00 lor\% ipsumlor emi 0.00 lore ipsumlor ip 0.0 lo remipsu mlo 00 l lo remi psumloremip.
\item [b] Loremipsum lor emipsu LORE.
\item [c] Loremipsum dol ORE.
\end{tablenotes}
\end{threeparttable}
\end{table}

\end{document}

Answer 1

The following solution achieves your formatting objective. Observe that it's not a good idea to execute \def\TPTnoteSettings{\setlength\labelwidth{1em}}, as doing so clobbers the default setting of the macro. That's why the code below contains the instruction

\apptocmd{\TPTnoteSettings}{\setlength\labelwidth{1em}}{}{}

In the code below, I've also cleaned up and simplified your preamble code. By the way, the booktabs package is utterly unrelated to "support centering of table". Instead, it provides macros -- such as \toprule and \bottomrule -- to draw well-spaced horizontal lines. Note also that the \begin{threeparttable} instruction should come before, not after, the \caption statement.

\documentclass[12pt,english]{article}
\renewcommand{\familydefault}{\sfdefault}
\usepackage{babel}
\usepackage[T1]{fontenc}
\usepackage[utf8]{inputenc} % "latin9" -- are you serious ?!
\usepackage[verbose,vmargin=2.5cm,lmargin=3cm,rmargin=2.5cm]{geometry}

\setlength{\parskip}{\medskipamount}
\setlength{\parindent}{0pt}

\usepackage{array} % for '\newcolumntype' macro
\newcolumntype{M}[1]{>{\centering}m{#1}}

\usepackage{setspace}
\onehalfspacing

\providecommand\tabularnewline{\\} % just in case it isn't already defined

\usepackage{caption} 
\captionsetup{skip=0.333\baselineskip,
              font=footnotesize % really?
             }
             
\usepackage{booktabs} % booktabs: to support centering of table
    %% No!! 'booktabs' is utterly unrelated to centering a table.
    %% Who on earth gave you this erroneous piece of information?
    
\usepackage{etoolbox} % for '\AtBeginEnvironment' macro and '\apptocmd' macro
\usepackage{threeparttable}
\AtBeginEnvironment{tablenotes}{\footnotesize\smallskip} 
\def\tnote#1{\protect\TPToverlap{\textsuperscript{[\TPTtagStyle{#1}]}}} % cf. https://tex.stackexchange.com/a/578489
\apptocmd{\TPTnoteSettings}{\setlength\labelwidth{1em}}{}{} % <-- new


\begin{document}
\begin{table}
\centering
\begin{threeparttable} % <-- should come _before_ '\caption'
\caption{}

\begin{tabular}{@{} M{1.2cm} M{4cm} M{3cm} M{1.8cm} M{1.9cm} @{}}
\toprule
1\tnote{a} & 2\tnote{b} & 3 & 4 & 5\tnote{c} \tabularnewline 
\bottomrule
\end{tabular}

\begin{tablenotes}
\item[a] Loremips umdolor sit ahmet 0.00 lore mipsum, 00 lor\% ipsumlor emi
         0.00 lore ipsumlor ip Lo lo remipsu mlo 00 l lo remi psumloremip.
\item[b] Loremipsum lor emipsu LORE.
\item[c] Loremipsum dol ORE.
\end{tablenotes}
\end{threeparttable}
\end{table}

\end{document}