It works better than with the weird hover behaviour i could do in CSS,
and it already has most of the aria roles needed.
The only tricky part is to allow closing it by clicking anywhere else,
that is done by “extending” the <summary> to the whole screen, with a
lower z-index than the menu but higher than the rest of controls, that
way we force people to click on that summary.
Had to call xgettext on Go source files because now the title comes from
there, as i assume i will have titles like "Invoice #INVxxxx" that have
to come from the database that the template does not know.