In VSCode, the new Sticky Scroll feature doesn't seem to work out of the box with my custom language extension. Is there some interface my language needs to implement in order to support it?
The new Sticky Scroll feature seems to be based on the language elements (class/interface/namespace/function/method/constructor) being recognized, and available in the Outline view. This means your custom language must have a Language Server or any other tooling that provides such elements to the editor.
If your language does provide that, but is not being properly supported in the new Sticky Scroll feature, I suggest you to open an issue in VS Code repo. As you can see (https://github.com/microsoft/vscode/labels/editor-sticky-scroll), there are a few issues reported.
Hope this helps