Indexing is quite expensive, so the goal for an usable indexing engine is to avoid it if nothing changes. If we can detect that we don't need to re-index that file. But not only file changes can change the index, project file changes too. To make it more complicated the preprocessor can change the index in a hard to detect way too. We have to find a compromise between usability and speed in that cases. For that we have to document the support and unsupported cases.