I'm trying to read the source of Beamer default outer theme (the file beamerouterthemedefault.sty). In line 144 there is this code:
\defbeamertemplate*{frametitle}{default}[1][left]
{
\ifbeamercolorempty[bg]{frametitle}{}{\nointerlineskip}%
% more things here
}
My question is simple: what is the purpose of this \nointerlineskip, and how is this connected to the background of the frametitle Beamer-color?
Edit: I checked on the repository on Bitbucket, and couldn't find any hints there (I hoped for some enlightening commit message found through "blame" - but no, nothing.)
\nointerlineskipis useful, for example, when one wants to place two boxes (in vertical mode) one exactly above the other, with no glue between them. – egreg Mar 13 at 9:52\nointerlineskipsuppresses\topskip? (I don't think so.) (2) maybebeamercolorboxputs some kind of a box above its contents (probably yes, though it'll take some time for me to analyse its code). – mbork Mar 13 at 10:00