Gnucash 3.3 on Ubuntu 18.04 and 18.10 Gnucash appears to impose a minimum window width such that the window cannot be shrunk below this size. If the available display width is less than that width then it is impossible to maximise the window. Version 2.6.19 also imposed a minimum width but it was several hundred pixels smaller so (at least for me) this was not an issue. The algorithm used to determine the min width is not obvious, it changes dependent on which accounts are open, but not in an obvious way. I don't see any reason why the limit is so large, I don't see why it was as large as it was on 2.6.19 either.
For the record I was typically seeing a minimum window width of 1350px with 3.3 whereas with 2.6.19 it was typically about 1000 px.
Having updated to Gnucash 3.6 (now on Ubuntu 19.04) this has been much improved. I am now seeing a minimum width of about 980px.
This should be fixed and in the next release 3.9
Thank you very much for fixing this issue!