When using kpfonts, the vector versions of i and j don't behave nice with the physics package

Question

When attempting to typeset unit vectors with the physics package shorthand \vu the following problems occur when using kpfonts which do not occur when using computer modern.

1. \vu{i} and \vu{j} do not use dotless versions of i and j.
2. When explicitly stating \vu{\imath} and \vu{\jmath}, the produced output is not in boldface, like every other character.
3. When working around the issue using \hat{\textbf{\j}}, the output (just for j) still has a dot.

Answer 1

If I try

\documentclass{article}
%\usepackage{kpfonts}
\usepackage{physics}

\begin{document}

$\vu{i}+\vu{j}$

\end{document}

(note that kpfonts is commented out), I get

The dots are there and it's not surprising, because the package does

% Unit vector [star for Greek and italic Roman]
\DeclareDocumentCommand\vectorunit{ s m }{%
  \IfBooleanTF{#1}
    {\boldsymbol{\hat{#2}}}
    {\mathbf{\hat{#2}}}%
}
% Shorthand for \vectorunit
\DeclareDocumentCommand\vu{}{\vectorunit}

(reformatted for clarity).

Here's a way out (not only for kpfonts):

\documentclass{article}
\usepackage{kpfonts}
\usepackage{physics}

\makeatletter
\ifkp@sfmath
  \DeclareSymbolFont{boldletters}{OT1}{jkpss\kp@fligm\kp@stylem}{b}{n}
  \SetSymbolFont{boldletters}{bold}{OT1}{jkpss\kp@fligm\kp@stylem}{b}{n}
\else
  \DeclareSymbolFont{boldletters}{OT1}{jkp\kp@famillem\kp@fligm\kp@stylem}{b}{n}
  \SetSymbolFont{boldletters}{bold}{OT1}{jkp\kp@famillem\kp@fligm\kp@stylem}{b}{n}
\fi
\SetSymbolFont{boldletters}{rm}{OT1}{jkp\kp@famillem\kp@fligm\kp@stylem}{b}{n}
\SetSymbolFont{boldletters}{boldrm}{OT1}{jkp\kp@famillem\kp@fligm\kp@stylem}{b}{n}
\SetSymbolFont{boldletters}{sf}{OT1}{jkpss\kp@fligm\kp@stylem}{b}{n}
\SetSymbolFont{boldletters}{boldsf}{OT1}{jkpss\kp@fligm\kp@stylem}{b}{n}
\DeclareSymbolFontAlphabet{\mathbf}{boldletters}
\makeatother
\DeclareMathSymbol{\bi}{\mathord}{boldletters}{"10}
\DeclareMathSymbol{\bj}{\mathord}{boldletters}{"11}

\begin{document}

$\vu{\bi}+\vu{\bj}$

\textbf{i j \^{\i} \^{\j}}

i j

\end{document}

A “self-fixing" approach, so you can input \vu{i} and \vu{j}.

\documentclass{article}
\usepackage{kpfonts}
\usepackage{physics}

\makeatletter
\ifkp@sfmath
  \DeclareSymbolFont{boldletters}{OT1}{jkpss\kp@fligm\kp@stylem}{b}{n}
  \SetSymbolFont{boldletters}{bold}{OT1}{jkpss\kp@fligm\kp@stylem}{b}{n}
\else
  \DeclareSymbolFont{boldletters}{OT1}{jkp\kp@famillem\kp@fligm\kp@stylem}{b}{n}
  \SetSymbolFont{boldletters}{bold}{OT1}{jkp\kp@famillem\kp@fligm\kp@stylem}{b}{n}
\fi
\SetSymbolFont{boldletters}{rm}{OT1}{jkp\kp@famillem\kp@fligm\kp@stylem}{b}{n}
\SetSymbolFont{boldletters}{boldrm}{OT1}{jkp\kp@famillem\kp@fligm\kp@stylem}{b}{n}
\SetSymbolFont{boldletters}{sf}{OT1}{jkpss\kp@fligm\kp@stylem}{b}{n}
\SetSymbolFont{boldletters}{boldsf}{OT1}{jkpss\kp@fligm\kp@stylem}{b}{n}
\DeclareSymbolFontAlphabet{\mathbf}{boldletters}
\makeatother
\DeclareMathSymbol{\bi}{\mathord}{boldletters}{"10}
\DeclareMathSymbol{\bj}{\mathord}{boldletters}{"11}

\ExplSyntaxOn
\RenewDocumentCommand\vectorunit{ s m }
 {
  \IfBooleanTF{#1}
    {\boldsymbol{\hat{#2}}}
    {\mathbf{\hat{\fixphysics_replace:n{#2}}}}
 }
\cs_new:Nn \fixphysics_replace:n
 {
  \str_case:nnF { #1 }
   {
    { i } { \bi }
    { j } { \bj }
   }
   { #1 }
 }
\ExplSyntaxOff

\begin{document}

$\vu{i}+\vu{j}$

\textbf{i j \^{\i} \^{\j}}

i j

\end{document}

Answer 2

While \j does have a dot, is it outside of the character boundary, so all one must do is explicitly clip to the font bounding box.

\usepackage{kpfonts}
\usepackage{amsthm, amsmath, amssymb}
\usepackage{mathrsfs}
\usepackage{trimclip}
\usepackage{physics}

% HACK: \imath and \jmath don't go bold in the \vu environment, so use text mode instead
\newcommand{\ihat}{\, \hat{\textbf{\i}}}
% HACK: \j still has a dot, but it's defined outside of the bounding box for the character. Explicitly clip so it doesn't display.
\newcommand{\jhat}{\, \hat{\textbf{\clipbox{0pt 0pt 0pt 0pt}{\j}}}}