I think you are probably referring to the old WinHelp style Non-Scrolling Region?
Framesets are used for this or you can do it using CSS and an "overflow" attribute as talked about in the link below.
Helpful and Handy Links
Yes, I inherited a winhelp project that used Non-scrolling regions for some of the topics (now that I know what they are called!), and I had to convert it to HTML help. Other help projects that I've worked on did not use them. And of course when I did the conversion, they did not map over to the HTML version.
So thank you very much for the information, I will pursue both avenues you suggest!