fix(build): watch preprocessor dependencies during build --watch (#653)

Author: dominikg

.changeset/sweet-fireants-attend.md

@@ -0,0 +1,5 @@
+---
+'@sveltejs/vite-plugin-svelte': patch
+---
+
+fix(build): watch preprocessor dependencies during build --watch

packages/vite-plugin-svelte/src/index.ts

@@ -236,10 +236,16 @@ export function svelte(inlineOptions?: Partial<Options>): Plugin[] {
 				}
 				logCompilerWarnings(svelteRequest, compileData.compiled.warnings, options);
 				cache.update(compileData);
-				if (compileData.dependencies?.length && options.server) {
-					compileData.dependencies.forEach((d) => {
-						ensureWatchedFile(options.server!.watcher, d, options.root);
-					});
+				if (compileData.dependencies?.length) {
+					if (options.server) {
+						for (const dep of compileData.dependencies) {
+							ensureWatchedFile(options.server.watcher, dep, options.root);
+						}
+					} else if (options.isBuild && viteConfig.build.watch) {
+						for (const dep of compileData.dependencies) {
+							this.addWatchFile(dep);
+						}
+					}
 				}
 				log.debug(`transform returns compiled js for ${svelteRequest.filename}`);
 				return {