On the left (obtained from the MWE below), I do not use boxes for the title nor the body text and the title and the text appears to be in the correct spot.

However, when I attempt to use newsaveboxs (MWE with defUseBoxes{} uncommented) I obtain the results on the right.

Questions:

• What are this additional skip's being inserted and how do I get both to exhibit identical behavior?


• In my actual use case I can tweak the addition of the struts to get correct behavior, but that also seems to need a conditional based on if the body text is more than one line. My understanding is that I should only need a strut at the end of the body text, but that does not seem to work here.

Notes:

• A vertical space defined by DesiredSkipAboveTitle is added before the title and a DesiredSkipBelowTitle is added after the title.


• Need to run at least twice to get the tikz drawing in correct position.


• The adjustbox package can be used to move the boxes into what I think is the correct position. This can be seen with both defUseBoxes{} and defUseAdjustBox{} uncommented. From this we see that the TitleBox needs a raise=-1.3pt option and the BodyBox requires a raise=-13.3pt with the align=t. I would like to know where these amounts come from.

Code:

defUseBoxes{}% Uncomment to use boxes

%defUseAdjustBox{}% Uncomment to use adjustbox (with added fudge factors)

%% --------------------

documentclass{article}

usepackage{tikz}

usepackage{tikzpagenodes}

usepackage[export]{adjustbox}



usepackage[paperwidth=7.0cm,showframe]{geometry}



newcommand*{DesiredSkipAboveTitle}{5pt}

newcommand*{DesiredSkipBelowTitle}{10pt}



newcommand*{NumberOfTitleLines}{2}

newcommand*{Title}{A Title that Takes Up Two Lines}

newsavebox{TitleBox}

newcommand*{SetupTitleBox}[2]{%

    setbox#1vbox{%

        bfseriescentering%

        #2%

        %strut%

        par%

    }%

}



newcommand*{BodyTextLarge}{%

    Lorem ipsum dolor sit amet, consectetur adipiscing elit. 

    Integer id nisl porta, porttitor tellus sed, sollicitudin erat. 

    Aliquam tellus urna, feugiat et tortor quis, aliquam mattis orci.% 

}%

newcommand*{BodyTextSmall}{%

    Small Text.% 

}%



newcommand{SelectedBodyText}{BodyTextLarge}

%newcommand{SelectedBodyText}{BodyTextSmall}





newsavebox{BodyBox}

newcommand{SetupBodyBox}[2]{%

    setbox#1vbox{%

        noindent#2%

        %strut%

    }%

}%



newlength{AdditionalSkip}

newcommand*{ShowTextGuideLines}[1]{%

begin{tikzpicture}[remember picture, overlay]

    coordinate (X) at (current page text area.north west);



    draw [draw=red, fill=yellow, fill opacity=0.2] 

        (X) circle (1pt);%% DEBUGGING: Ensure (X) is the correct spot.



    setlength{AdditionalSkip}{DesiredSkipAboveTitle}%

    foreach X in {1, ..., #1} {%

        ifnumX>NumberOfTitleLines

            %% After title: need to adjust AdditionalSkip for space after title

            setlength{AdditionalSkip}{%

                dimexprDesiredSkipAboveTitle+DesiredSkipBelowTitlerelax%

             }%

        fi

        draw [thin, red] ([yshift=-Xbaselineskip-AdditionalSkip]X) -- ++ (hsize,0);

    }%



    tikzset{Node Style/.style={anchor=south, draw=red, inner sep=1pt}}

    ifdefinedUseBoxes

        node [Node Style, fill=yellow] at ([xshift=0.5hsize]X) {using boxes};

    else

        node [Node Style, fill=green] at ([xshift=0.5hsize]X) {Not using boxes};

    fi

end{tikzpicture}%

}%





begin{document}

    %% ---------------------------------------------------------- Set up the title and body

    SetupTitleBox{TitleBox}{Title}%

    SetupBodyBox{BodyBox}{SelectedBodyText}%

    %% ---------------------------------------------------------- Title

    hbox{}kern-topskip%% See https://tex.stackexchange.com/questions/7676/why-does-vspace0pt-add-vertical-space#comment12433_7681

    vspace*{DesiredSkipAboveTitle}%

    %%

    ifdefinedUseBoxes

        ifdefinedUseAdjustBox% Following tweaks seem to be close

            noindentadjustbox{valign=t,raise=-1.3pt}{usebox{TitleBox}}%

            vspace*{DesiredSkipBelowTitle}%

            parnoindentadjustbox{valign=t,raise=-13.3pt}{usebox{BodyBox}}%

        else

            noindentusebox{TitleBox}%

            vspace*{DesiredSkipBelowTitle}%

            parnoindentusebox{BodyBox}%

        fi

    else

        noindent{bfseriescenteringTitlepar}%

        vspace*{DesiredSkipBelowTitle}%

        parnoindentSelectedBodyText%

    fi

    noindentShowTextGuideLines{7}%

end{document}

I think it would probably be useful to understand how TeX normally inserts vertical space between lines/boxes.
This will make it clear why this spacing is different when you wrap your paragraphs in a vbox, as in your MWE.
A method to achieve identical spacing with and without boxes can be found at the bottom this answer.

About baselineskip and lineskip

TeX normally tries to insert an appropriate amount of space between lines so that subsequent baselines are separated by baselineskip.
If it fails to do so without causing the distance between the boxes containing these lines to fall below lineskiplimit it'll insert lineskip instead.
Since lineskiplimit=0pt by default, this happens precisely when the boxes would otherwise overlap.

Because baselineskip=12pt, and lineskiplimit=0pt by default for a 10pt document, this means that if the distance between consecutive baselines is 12pt if the sum of the depth of the top line and the height of the bottom line is less than 12pt. Otherwise, the (boxes containing these) lines will be separated by lineskip (which is 1pt by default).

Here's an illustration:

parskip=0pt parindent=0pt

An ordinary line of text

par

Another ordinary line of text



vspace{baselineskip}



rule[-5pt]{10pt}{15pt}This line has a depth of texttt{5pt}

par

rule[-5pt]{10pt}{15pt}This line has a height of texttt{10pt}

Note that if you manually add additional skips, these are just added to whatever amount of space would otherwise be inserted:

parskip=0pt parindent=0pt

An ordinary line of text

parvspace{2mm}

Another ordinary line of text



vspace{baselineskip}



rule[-5pt]{10pt}{15pt}This line has a depth of texttt{5pt}

parvspace{2mm}

rule[-5pt]{10pt}{15pt}This line has a height of texttt{10pt}

The same thing, but with vertical boxes

It is no different for your vboxes. A vbox containing multiple lines has just a single baseline, which coincides with the baseline of the bottom line of its contents (top line for a vtop).
If TeX can't separate these baselines by exactly baselineskip without the boxes overlapping, it will completely ignore baselineskip, put a vertical space of lineskip between these boxes and call it a day.

This is the result: (green lines indicate the baselines)

parskip=0pt parindent=0pt

vbox{The first vbox containing\two lines}

par

vbox{The first vbox containing\two lines}

So the distance between the baseline of the second line of the first box and the baseline of the first line of the second box is equal to the depth of the former line plus the height of the latter line plus lineskip.
In this case I think that's 0pt + 6.94444pt + 1pt = 7.94444pt (the second number is how tall the letters h, d and b are).
Adding a strut or some character with descenders (gjpqy) to the first vbox would increase this distance by increasing the depth of the last line of this box.

The same thing happens between consecutive vtops:

parskip=0pt parindent=0pt

vtop{The first vtop containing\two lines}

par

vtop{The first vtop containing\two lines}

The only situation that likely matches your expected/desired outcome is the following:

parskip=0pt parindent=0pt

vbox{A vbox containing\two lines}

par

vtop{A vtop containing\two lines}

This happens because the baselines are close enough together that they can be separated by exactly baselineskip without the boxes overlapping.
The space above this vbox and below this vtop is again going to be "wrong", however.

The top of the page

Something similar happens at the top of the page.
TeX tries to insert a (positive) skip that wil separate the baseline of the first box on a page from the top of the text area by topskip. If it can't, it'll just place the box directly at the top.
The default value of topskip is equal to the font size (so 10pt for a 10pt document).

This is why it matters whether you put a vtop or a vbox at the top of your page.

documentclass{article}

usepackage[showframe]{geometry}

begin{document}

  parskip=0pt parindent=0pt

  vtop{A vtop containing\two lines}

  clearpage

  vbox{A vbox containing\two lines}

end{document}

In your MWE topskip isn't really relevant because you've inserted hbox{}kern-topskip. The hbox creates an empty box that will be separated from the top of the text area by the full topskip, and the subsequent kern undoes this skip.
It is thus as if there is an invisible paragraph at the top of your document that ends where the text area begins, so TeX tries to separate the first box (after this) from the top of the text area by baselineskip and insert lineskip if it can't, exactly as described above.

A possible solution

To get the correct spacing above a vertical box you should use vtop instead of vbox for the reasons outlined above.

To get the right amount of spacing below a vtop you can set its depth to zero and then insert a vertical skip just below the box equal to <n>baselineskip, where <n> is the number of lines in the box beyond the first. This skip can be calculated by taking the depth of the box and rounding down to a multiple baselineskip. You may want to set lineskiplimit=-maxdimen (inside the box, or globally) to ensure that all baselines inside the box are separated by exactly baselineskip.

The macro copyboxwithappropriatespace, which I define below (and which could use a better name), does almost precisely these two things. Instead of setting the depth to zero it sets it to the remainder of the division of the original depth by baselineskip (as suggested by jfbu in the comments).
This matters if the last line of your box is particularly deep or the first line of the next box is particularly high.

makeatletter                %% <-- make @ usable in command sequences

%% Inserts a box of depth 0 and insert an appropriate vspace:

newcommand*copyboxwithappropriatespace[1]{%

  begingroup

    @tempdima=dp#1relax   %% <-- store the depth in @tempdima

    count@=@tempdimarelax %% <-- convert to a number

    dividecount@ by baselineskip %% <-- divide by baselineskip

    dp#1=dimexpr@tempdima-count@baselineskip %% <-- set the depth to to the remainder

    copy#1%                 %% <-- insert the box

    dp#1=@tempdimarelax   %% <-- reset its depth

    vspace*{count@baselineskip}%

  endgroup                  %% ^-- insert count@ baselineskips

}

makeatother                 %% <-- revert @

This command replaces copy (or usebox).

(If you want, you could define useboxwithappropriatespace by copying this definition, removing dp#1=@tempdimarelax and replacing copy by usebox in this definition.)

documentclass{article}



usepackage{showframe}     %% <- show boundary of text area

usepackage{tikz}          %% <- To draw the red brackets

usepackage{tikzpagenodes} %% <- to place them correctly



makeatletter                %% <-- make @ usable in command sequences

%% Inserts a box of depth 0 and insert an appropriate vspace:

newcommand*copyboxwithappropriatespace[1]{%

  begingroup

    @tempdima=dp#1relax   %% <-- store the depth in @tempdima

    count@=@tempdimarelax %% <-- convert to a number

    dividecount@ by baselineskip %% <-- divide by baselineskip

    dp#1=dimexpr@tempdima-count@baselineskip %% <-- set the depth to to the remainder

    copy#1%                 %% <-- insert the box

    dp#1=@tempdimarelax   %% <-- reset its depth

    vspace*{count@baselineskip}%

  endgroup                  %% ^-- insert count@ baselineskips

}

makeatother                 %% <-- revert @



parindent=0pt %% <- saves me from having to type noindent

parskip=0pt   %% <- default value is "0pt plus 1pt"



begin{document}



newsavebox{TitleBox}

newsavebox{BodyBox}

setboxTitleBox=vtop{textbf{A title that takes\up more than\two lines}}

setboxBodyBox=vtop{Lorem\ipsum}



copyboxwithappropriatespaceTitleBox



copyboxwithappropriatespaceBodyBox



copyboxwithappropriatespaceBodyBox



copyboxwithappropriatespaceBodyBox



begin{tikzpicture}[remember picture, overlay,]

    draw[red] (current page text area.north west) -- +(-2pt,0)

                 -- node[left]{texttt{detokenize{topskip}}} +(-2pt,-topskip) -- +(0,-topskip);

    foreach X in {0, ..., 7} {

      draw[red] ([yshift=-topskip-Xbaselineskip]current page text area.north west) -- +(-2pt,0)

                 -- node[left]{texttt{detokenize{baselineskip}}} +(-2pt,-baselineskip) -- +(0,-baselineskip);

    }

end{tikzpicture}



clearpage



textbf{A title that takes\up more than\two lines}



Lorem\ipsum par Lorem\ipsum par Lorem\ipsum



end{document}

This produces two pages that look identical (apart from the fact that only the first has the red brackets) and which were constructed differently: one using vertical boxes and one using ordinary paragraphs.

Note that I've set parskip=0pt because the default value of parskip is not equal to 0pt but to 0pt plus 1pt.
This means that the amount of space between paragraphs can be stretched to up to 1pt to improve the page layout, which I'm guessing you don't want to happen.

In your MWE

The spacing in your MWE will be correct if you replace vbox by top and replace usebox by copyboxwithappropriatespace (and add the above definition to your preamble, of course):

defUseBoxes{}% Uncomment to use boxes

%% --------------------

documentclass{article}

usepackage{tikz}

usepackage{tikzpagenodes}



usepackage[paperwidth=7.0cm,showframe]{geometry}



newcommand*{DesiredSkipAboveTitle}{5pt}

newcommand*{DesiredSkipBelowTitle}{10pt}



newcommand*{NumberOfTitleLines}{2}

newcommand*{Title}{A Title that Takes Up Two Lines}

newsavebox{TitleBox}

newcommand*{SetupTitleBox}[2]{%

    setbox#1vtop{%

        bfseriescentering%

        #2%

        %strut%

        par%

    }%

}



newcommand*{BodyTextLarge}{%

    Lorem ipsum dolor sit amet, consectetur adipiscing elit. 

    Integer id nisl porta, porttitor tellus sed, sollicitudin erat. 

    Aliquam tellus urna, feugiat et tortor quis, aliquam mattis orci.% 

}%

newcommand*{BodyTextSmall}{%

    Small Text.% 

}%



newcommand{SelectedBodyText}{BodyTextLarge}

%newcommand{SelectedBodyText}{BodyTextSmall}





newsavebox{BodyBox}

newcommand{SetupBodyBox}[2]{%

    setbox#1vtop{%

        noindent#2%

        %strut%

    }%

}%



makeatletter                %% <-- make @ usable in command sequences

%% Inserts a box of depth 0 and insert an appropriate vspace:

newcommand*copyboxwithappropriatespace[1]{%

  begingroup

    @tempdima=dp#1relax   %% <-- store the depth in @tempdima

    count@=@tempdimarelax %% <-- convert to a number

    dividecount@ by baselineskip %% <-- divide by baselineskip

    dp#1=dimexpr@tempdima-count@baselineskip %% <-- set the depth to to the remainder

    copy#1%                 %% <-- insert the box

    dp#1=@tempdimarelax   %% <-- reset its depth

    vspace*{count@baselineskip}%

  endgroup                  %% ^-- insert count@ baselineskips

}

makeatother                 %% <-- revert @



newlength{AdditionalSkip}

newcommand*{ShowTextGuideLines}[1]{%

begin{tikzpicture}[remember picture, overlay]

    coordinate (X) at (current page text area.north west);



    draw [draw=red, fill=yellow, fill opacity=0.2] 

        (X) circle (1pt);%% DEBUGGING: Ensure (X) is the correct spot.



    setlength{AdditionalSkip}{DesiredSkipAboveTitle}%

    foreach X in {1, ..., #1} {%

        ifnumX>NumberOfTitleLines

            %% After title: need to adjust AdditionalSkip for space after title

            setlength{AdditionalSkip}{%

                dimexprDesiredSkipAboveTitle+DesiredSkipBelowTitlerelax%

             }%

        fi

        draw [thin, red] ([yshift=-Xbaselineskip-AdditionalSkip]X) -- ++ (hsize,0);

    }%



    tikzset{Node Style/.style={anchor=south, draw=red, inner sep=1pt}}

    ifdefinedUseBoxes

        node [Node Style, fill=yellow] at ([xshift=0.5hsize]X) {using boxes};

    else

        node [Node Style, fill=green] at ([xshift=0.5hsize]X) {Not using boxes};

    fi

end{tikzpicture}%

}%





begin{document}

    %% ---------------------------------------------------------- Set up the title and body

    SetupTitleBox{TitleBox}{Title}%

    SetupBodyBox{BodyBox}{SelectedBodyText}%

    %% ---------------------------------------------------------- Title

    hbox{}kern-topskip%% See https://tex.stackexchange.com/questions/7676/why-does-vspace0pt-add-vertical-space#comment12433_7681

    vspace*{DesiredSkipAboveTitle}%

    %%

    ifdefinedUseBoxes

        noindentcopyboxwithappropriatespace{TitleBox}%

        vspace*{DesiredSkipBelowTitle}%

        parnoindentcopyboxwithappropriatespace{BodyBox}%

    else

        noindent{bfseriescenteringTitlepar}%

        vspace*{DesiredSkipBelowTitle}%

        parnoindentSelectedBodyText%

    fi

    noindentShowTextGuideLines{7}%

end{document}