test(fmt): pin advertised LSP text document sync capability

Author: fi3ework

packages/rstack/src/fmt/lsp/server.ts

@@ -229,7 +229,9 @@ const startFmtLsp = (options: RunFmtLspOptions, onExit: () => void): void => {
 
     return {
       // The project config is the single source of truth, so client formatting
-      // options are ignored and no other capability is advertised.
+      // options are ignored. The connection fills in incremental
+      // `textDocumentSync` for the `TextDocuments` listener; nothing else is
+      // advertised.
       capabilities: { documentFormattingProvider: true },
     };
   });

packages/rstack/tests/cli/fmt/lsp.test.ts

@@ -48,6 +48,9 @@ test(
       const { capabilities } = await client.initialize();
 
       expect(capabilities.documentFormattingProvider).toBe(true);
+      // Incremental sync, filled in by the connection for the `TextDocuments`
+      // listener; without it compliant clients would never send the document.
+      expect(capabilities.textDocumentSync).toBe(2);
       expect(capabilities.documentRangeFormattingProvider).toBeUndefined();
       expect(capabilities.documentOnTypeFormattingProvider).toBeUndefined();
     });