Unless you are already familiar with GitHub, it's usually easiest to report problems through the discussion groups.
Open issues should be actionable: generally either a reproducible bug report, or a specific feature request. From the perspective of the core developers, the issues list behaves like a shared todo list. Every item on the list requires a little bit of attention each time we check the list.
GitHub Issues are not very good for managing ideas that are not immediately actionable. Better to use the TiddlyWiki discussion groups for open ended questions, or speculative discussions of new features.
Before creating a GitHub issue it is good etiquette to search through the existing issues to see whether the problem has already been reported. If a search isn't practical, don't worry too much; GitHub makes it easy to merge existing issues.
When you do create an issue, remember that for effective debugging, we need as much information as possible. At a minimum, please try to include:
Consider also adding screenshots if it makes things clearer.
There's a lot of good material on the web about bug reports: