Installing gambas3 from PPA and then Debian packages is not a common use case. If you do so, you are on your own and cannot expect support. Please try to stick to the issue's topic.
I will not try to have a different approach to documentation in Debian than upstream. If anybody wants to improve the situation, I am happy to take patches and even post them upstream if you do not know how to do that yourselves. So, my question rephrased: When you install the current testing gambas3-ide package (currently version 3.12.2-1), does downloading the offline documentation work without installing any additional packages? If not, which packages do you need to install on top?
