\if@minipage purpose in floats

Question

In floating environments such as the captions of figures, LaTeX has a \global\@minipagefalse, why is this necessary and what does this switch do?

\long\def\@makecaption#1#2{%
  \vskip\abovecaptionskip
  \sbox\@tempboxa{#1: #2}
  \ifdim \wd\@tempboxa >\hsize
    #1: #2\par
  \else
    \global \@minipagefalse
    \hb@xt@\hsize{\hfil\box\@tempboxa\hfil}%
  \fi
  \vskip\belowcaptionskip}

Answer 1

The @minipage switch is used by LaTeX to handle vertical spacing at the beginning of a minipage environment (or a similar construct). If it is true then \addvspace is not adding space. So if, for example, a minipage starts of with a list environment then the space normally added in front of the list (by \addvspace) is suppressed.

The switch will be set to true at the beginning of a minipage environment (or a similar construct) and is normally set to false by \everypar, i.e., the moment textual material appears in that "minipage".

Now in the case of the caption implementation, two different strategies are used: if the caption is wider than the current horizontal width, then it is re-set as a normal paragraph (so the switch would be set to false by \everypar). However is it is small then the box is used directly without triggering the paragraph builder. Therefore one has to set the switch manually in this case.

And the reason that it is done globally is just because the switch is always manipulated globally.

Answer 2

The sources say

The float mechanism, like minipage, sets the flag |@minipage| true
before executing the user-supplied text. Many \LaTeX\ constructs
test for this flag and do not add vertical space when it is true.
The intention is that this emulates \TeX's `top of page' behaviour.
The flag must be set false at the start of the first paragraph. This
is achieved by a redefinition of |\everypar|, but the call to
|\@parboxrestore| removes that redefinition, so it is re-inserted 
if needed. If the flag is already false then the |\caption| was not
the first entry in the float, and so some other paragraph has already
activated the special |\everypar|. In this case no further action is
needed.

The most important command that tests this switch is \addvspace which is the command that is mostly used to add the vertical space around display environments such as lists. It is a global flag and setting it in a float environment means the float acts in a consistent way even if it floats from somewhere where it is set to somewhere where it would not be set.