If \makedgapedcells switched on for the table there is compensate spacing.
297 {\ifx\@classz\mcell@classz
298 \setbox\z@\hbox{#1}\ht\z@\z@\dp\z@\z@
299 \mcell@MB@\z@\mcell@MBjot
300 \global\dimen@\@tempdima\global\@tempdimb\@tempdimb
301 \else\global\dimen@\z@\global\@tempdimb\z@\fi
302 }%
303 \advance\@tempdima\dimen@
304 \edef\mcell@diagVoffset{\the\@tempdima}%
305 \@tempdima\mcell@diagV\advance\@tempdima-\mcell@diagVoffset
306 \advance\@tempdima-\@tempdimb
307 \edef\mcell@diagVcorr{\the\@tempdima}%
308 \noindent\nomakegapedcells\hbox{\begin{tabular}{@{}c@{}}%
At least a \normallineskip vertical space from top and bottom of cell.
309 \ifdim\jot<2\p@\jot2\p@\fi
310 \if@tempswa
For South-East or North-West directions.
311 \begin{picture}(\mcell@diagH,\mcell@diagVcorr)(\z@,\mcell@diagVoffset)%
312 \put(\z@,\mcell@diagV){\makebox(\z@,\z@)[tl]%
313 {\edef\tempa{(\mcell@Hratio,-\mcell@Vratio)}\expandafter
314 \line\tempa{\mcell@diagH}}}
315 \put(\tabcolsep,\jot)%
316 {\makebox(\z@,\z@)[bl]{\theadfont
317 \let\cellset\theadset\makecell[bl]{\strut#2}}}
318 \@tempdima\mcell@diagH\advance\@tempdima-\tabcolsep
319 \@tempdimb\mcell@diagV\advance\@tempdimb-\jot
320 \put(\@tempdima,\@tempdimb)%
321 {\makebox(\z@,\z@)[tr]{\theadfont
322 \let\cellset\theadset\makecell[tr]{#3\strut}}}
323 \end{picture}%
324 \else
For South-West or North-East directions.
325 \begin{picture}(\mcell@diagH,\mcell@diagVcorr)(\z@,\mcell@diagVoffset)%
326 \put(\z@,\mcell@diagV){\makebox(\z@,\z@)[tl]%
327 {\edef\tempa{(\mcell@Hratio,\mcell@Vratio)}\expandafter
328 \line\tempa{\mcell@diagH}}}
329 \@tempdima\mcell@diagV\advance\@tempdima-\jot
330 \put(\tabcolsep,\@tempdima)%
331 {\makebox(\z@,\z@)[tl]{\theadfont
332 \let\cellset\theadset\makecell[tl]{\strut#3}}}
333 \@tempdima\mcell@diagH\advance\@tempdima-\tabcolsep
334 \put(\@tempdima,\jot)%
335 {\makebox(\z@,\z@)[br]%
336 {\theadfont
337 \let\cellset\theadset{\makecell[br]{#2\strut}}}}
338 \end{picture}%
339 \fi
340 \end{tabular}%
33