Thanks for the suggestion. I have removed all the trailing @tab, and space between rows is now consistent. It looks like a workaround to me, but it's effective. However, the space between the title and the first row is smaller than the one between rows. And, imho, it does not look very nice:
In Info, I also think it's a bit too close, but it's ok (thanks to the rule).
Long Option Short Option Bison Directive
---------------------------------------------------------------------------------
‘--color[=WHEN]’
‘--debug’ ‘-t’ ‘%debug’
‘--define=NAME[=VALUE]’ ‘-D NAME[=VALUE]’ ‘%define NAME [VALUE]’
In HTML, default CSS, it looks right to me.
I can live with that, of course. I just meant to report it, as maybe at least the default TeX rendering might not be the one the Texinfo developers wanted.
Cheers!