Summary

Creating a brand‑new component template whose name ends with @custom can immediately show the “Apply mappings now and rollover?” modal listing many existing data streams (often Fleet streams), even though the new component isn’t referenced by any index template and may be empty or settings‑only.

Expected

• No modal unless the component template is actually referenced by an index template and an in‑place mapping apply fails for those data streams.

Actual

• A modal appears and lists many unrelated, pre‑existing data streams after creating an unused *@custom component template.

Why it happens (suspected)

• The “impacted data streams” API returns data streams even when the component template isn’t referenced by any index template (effectively querying all visible data streams).

  kibana/x-pack/plugins/index_management/server/routes/api/component_templates/register_datastream_route.ts

  Line 39 in 33393b9

  if (datastreamNames.length < 0) {

  
  Kibana then attempts to apply mappings to those streams; if any apply fails, the rollover modal is shown.

Impact

• Confusing user experience on Create and risk of unintended rollovers if “Apply now and rollover” is chosen.

Workarounds

• Choose “Apply on next rollover” (safe if the component is truly unused), or
• Create the component template with a name that does not end with @custom.

Proposed fix

• In the “impacted data streams” route, return an empty list when no index templates reference the component template, and only attempt apply‑mappings for legitimately impacted data streams.