Using \color inside a \parbox causes vertical displacement

Question

When modifying my answer to this question I came across some odd behaviour. If \color{<color>} is used at the start of a top-aligned \parbox, then the entire box is moved downwards, like this:

\documentclass{article}
\usepackage{xcolor}
\begin{document}
Word
\framebox{\parbox[t]{1cm}{word}} %% <- this is fine
\framebox{\parbox[t]{1cm}{\textcolor{blue}{word}}} %% <- this is fine
\framebox{\parbox[t]{1cm}{\color{blue}word}} %% <- ??
\framebox{\parbox[t]{1cm}{\hskip0pt\color{blue}word}} %% <- this is fine
\framebox{\parbox[t]{1cm}{\leavevmode\color{blue}word}} %% <- this is fine
\framebox{\parbox[t]{1cm}{$\displaystyle\int$}} %% <- this is fine
\framebox{\parbox[t]{1cm}{\color{blue}$\displaystyle\int$}} %% <- ??
\end{document}

(The \frameboxes are just there to show the locations/sizes of the \parboxes.)

Can someone tell me why this happens (and how I can best avoid it)?

Answer 1

\color is usually implemented via "whatsits", i.e. \special or \pdfcolorstack in case of pdfTeX. It becomes the top element at the start of the vertical box (\parbox[t]). The following paragraph starts below. Putting it after \leavevmode at the baseline of the starting paragraph is the right fix.

For debugging, a box can be inspected by \showbox:

\documentclass{article}
\usepackage{color}
\begin{document}
  \showboxdepth=\maxdimen
  \showboxbreadth=\maxdimen
  \tracingonline=1
  \setbox0=\hbox{\parbox[t]{1cm}{\color{blue}word}}
  \showbox0
\end{document}

The console (with \tracingonline=1) and the .log file reports:

\hbox(0.0+6.94444)x28.45274
.\vbox(0.0+6.94444)x28.45274
..\pdfcolorstack 0 push {0 0 1 rg 0 0 1 RG}
..\glue(\parskip) 0.0
..\hbox(6.94444+0.0)x28.45274, glue set 7.03604fil
...\hbox(0.0+0.0)x0.0
...\OT1/cmr/m/n/10 w
...\kern-0.27779
...\OT1/cmr/m/n/10 o
...\OT1/cmr/m/n/10 r
...\OT1/cmr/m/n/10 d
...\penalty 10000
...\glue(\parfillskip) 0.0 plus 1.0fil
...\glue(\rightskip) 0.0
..\pdfcolorstack 0 pop

! OK.
l.8   \showbox0