Safeguard an environment against tabular mode

Question

The semantics of a given environment require that it is typeset inside a minipage if it is used inside a tabular. If I add the minipage declaration for a positive check for "internal" mode in the environment using \ifinner, it works: the minipage is added only if in tabular mode. (This can be seen in the output because the minipage does not expand over the full text width.) I have the following questions:

1. Why does the check for inner mode succeed when the kframe environment is closed? Is TeX in "internal" mode inside a minipage environment?
2. Is there a more reliable way to do this detection? Is the detection correct at all?

The kframe environment is used in code generated by knitr; it should work both inside and outside of a tabular.

\documentclass{article}
\pagestyle{empty}

\usepackage{color}
\usepackage{framed}

\newenvironment{kframe}{%
  % The \ifinner check below is necessary
  \ifinner\begin{minipage}{.8\textwidth}\else\fi%
  \def\FrameCommand##1{\colorbox{yellow}{##1}}%
  \MakeFramed{}}{%
  \endMakeFramed%
  % Why does this \ifinner check work?
  \ifinner\end{minipage}\fi}

\begin{document}

\begin{kframe}
  Any text long enough will do here.
  Any text long enough will do here.
  Any text long enough will do here.
\end{kframe}

\begin{tabular}{l}
\begin{kframe}
  Any text long enough will do here.
  Any text long enough will do here.
  Any text long enough will do here.
\end{kframe}
\end{tabular}

\end{document}

Answer 1

It would be very peculiar to need a minipage inside a tabular, there is virtually nothing about a tabular cell which isn't shared by \mbox (for l columns) or \parbox (for p columns.).

It looks to me like you need a minipage if in an l column or \mbox or other restricted horizontal mode so I think you want

  \ifinner\ifhmode  start minipage \fi\fi

This would start a minipage in a tabular l but not a tabular p. The code below contains the full implementation including egreg's remark on closing the minipage environment.

\makeatletter
\newenvironment{kframe}{%
  \def\at@end@of@kframe{}%
  \ifinner\ifhmode%
    \def\at@end@of@kframe{\end{minipage}}%
    \begin{minipage}{.8\textwidth}%
  \fi\fi%
  \def\FrameCommand##1{\colorbox{yellow}{##1}}%
  \MakeFramed{}}{%
  \endMakeFramed%
  \at@end@of@kframe}
\makeatother