I am viewing this document with Firefox's internal pdf viewer, which results in
opening all external links in the same tab as the document itself.
See https://en.wikibooks.org/wiki/LaTeX/Hyperlinks#Customization for the option
pdfnewwindow "define if a new window should get opened when a link leads out of
the current document".
I am not sure of one should add this option to
fonts/opensans/doc/fonts/opensans/opensans.tex
too.
%\usepackage{draftwatermark}
\usepackage{gitinfo}
\usepackage{catchfile}
-\usepackage{hyperref}
+\usepackage[pdfnewwindow]{hyperref}
% for development
\usepackage{ifdraft}