For a custom language I created a CompletionContributor. Everything works fine. But I'm limited to the information I can display (only in the list or the bottom "advertisement" and only 1 line).
When trying to auto-complete on a java class name it will display more information on the selected line in a small side window. I would like to exploit that mechanism but I really don't know how it is done.
When looking at the options provided, I can use a custom LookupElementRenderer but there is no method in LookupElementPresentation related to the right window.
Any idea how it is done?
Are you referring to documentation popup which may be displayed if the corresponding setting (Setting->Editor->General->Code completion->Show the documentation popup in) is turned on?
If you want the feature to work for your language you have to use lang.documentationProvider extension point.