Reindex a library from code works even on a no-script site — the root folder's property bag is writable, the web's is not

Symptom

An admin tool lists slow or failed crawl entries and you want a "reindex this library" action next to them. Two things look like blockers:

• the crawl log row carries only a URL (its AccessData GUID belongs to the item, not to the list), so there is no list ID to build advsetng.aspx?List={id} from;
• the well-known observation that property-bag writes are blocked on no-script sites (true for web.AllProperties) suggests the reindex flag cannot be set from code either.

Cause

The no-script restriction is about the web property bag. The reindex flag lives on a different object — the list's root folder — and that write is allowed for anyone who can manage the list. Measured on a modern site with custom scripts disabled, as a site admin: the CSOM call returned ErrorInfo: null and the value was persisted.

GetList is more forgiving than its name suggests: it resolves the list from the root URL, a view or form page (Forms/AllItems.aspx, DispForm.aspx), a file deep in folders, or a crawl-log item URL such as …/Lists/Tasks/1_.000. It fails (404) only when the path has a query string, is still URL-encoded, or is sent to a parent web.

Fix

1. Strip ?… and #…, decode the path.
2. POST {anyWeb}/_api/SP.Web.GetWebUrlFromPageUrl with body { "pageFullUrl": "<absolute url>" } → the web the item lives in.
3. GET {web}/_api/web/GetList(@a)?@a='<server-relative path>'&$select=Id,Title,ItemCount → the library.
4. Read the current value: GET {web}/_api/web/lists(guid'<id>')/RootFolder/Properties?$select=vti_x005f_searchversion (missing = never reindexed, treat as 0).
5. Write n + 1 through {web}/_vti_bin/client.svc/ProcessQuery:

<Request SchemaVersion="15.0.0.0" LibraryVersion="16.0.0.0" ApplicationName="MyTool"
         xmlns="http://schemas.microsoft.com/sharepoint/clientquery/2009">
  <Actions>
    <Method Name="SetFieldValue" Id="10" ObjectPathId="6">
      <Parameters>
        <Parameter Type="String">vti_searchversion</Parameter>
        <Parameter Type="Int32">5</Parameter>
      </Parameters>
    </Method>
    <Method Name="Update" Id="11" ObjectPathId="5" />
  </Actions>
  <ObjectPaths>
    <StaticProperty Id="1" TypeId="{3747adcd-a3c3-41b9-bfab-4a64dd2f1e0a}" Name="Current" />
    <Property Id="2" ParentId="1" Name="Web" />
    <Property Id="3" ParentId="2" Name="Lists" />
    <Method Id="4" ParentId="3" Name="GetById">
      <Parameters><Parameter Type="Guid">{LIST-ID}</Parameter></Parameters>
    </Method>
    <Property Id="5" ParentId="4" Name="RootFolder" />
    <Property Id="6" ParentId="5" Name="Properties" />
  </ObjectPaths>
</Request>

CSOM reports errors inside an HTTP 200 body — check ErrorInfo of the response array, not the status code.

Notes

• The crawler picks the flag up on its next pass; SharePoint does not say when. Show "reindex requested", not "reindexed".
• A full reindex of a large library is real load on the tenant's crawl capacity. Confirm with the item count and do not offer it as a bulk action.
• If you open the settings page instead, call window.open synchronously inside the click and set its location after the lookup — a window opened after an await is dropped by popup blockers.