fix: pre-bundle @solidjs/signals when the app depends on it directly

.changeset/signals-optimize-include.md

@@ -0,0 +1,12 @@
+---
+'@solidjs/vite-plugin': patch
+---
+
+Pre-bundle `@solidjs/signals` up front under `vite dev` (and Vitest browser
+mode) when the app can import it by name, i.e. it resolves from the project
+root. The first optimizer pass otherwise inlines signals into `solid-js`, so a
+direct `@solidjs/signals` import from a module the scanner never reached (a
+browser-mode test file, a non-literal dynamic import) was a late discovery:
+re-optimize and a full page reload ("Vite unexpectedly reloaded a test").
+Apps that only get signals transitively (pnpm's isolated layout) are
+unchanged.

src/index.ts

@@ -1517,14 +1517,12 @@ export default function solidPlugin(options: Partial<Options> = {}): Plugin[] {
       // `solid-js` fails with ERR_MODULE_NOT_FOUND as soon as anything
       // externalizes it (vitefu does, once a semi-framework package such as
       // `@solidjs/diagnostics` lists it under `dependencies`). With no root
-      // copy there is nothing to dedupe TO, so the gate loses nothing. Not
-      // added to `optimizeDeps.include`: the optimizer already reaches
-      // signals through `solid-js`, and an include entry that doesn't
-      // resolve from the root logs a warning on every start.
-      const dedupe =
-        replaceDev && findPackageDir('@solidjs/signals', path.resolve(projectRoot || process.cwd()))
-          ? [...nestedDeps, '@solidjs/signals']
-          : nestedDeps;
+      // copy there is nothing to dedupe TO, so the gate loses nothing.
+      const rootSignals = !!findPackageDir(
+        '@solidjs/signals',
+        path.resolve(projectRoot || process.cwd()),
+      );
+      const dedupe = replaceDev && rootSignals ? [...nestedDeps, '@solidjs/signals'] : nestedDeps;
 
       const userTest = (userConfig as any).test ?? {};
       const test = {} as any;
@@ -1664,6 +1662,14 @@ export default function solidPlugin(options: Partial<Options> = {}): Plugin[] {
             ...(command === 'serve'
               ? ['solid-js/attribution', '@solidjs/web/performance-tracks']
               : []),
+            // An app that depends on `@solidjs/signals` directly can import
+            // it from a module the scanner never reaches (a test file in
+            // Vitest browser mode, a non-literal dynamic import). The first
+            // pass bundled signals inside `solid-js`, so that import is a
+            // late discovery: re-optimize, full reload. Only when it resolves
+            // from the root: otherwise the app can't import it by name, and
+            // an include entry that doesn't resolve warns on every start.
+            ...(command === 'serve' && rootSignals ? ['@solidjs/signals'] : []),
             ...solidPkgsConfig.optimizeDeps.include,
           ],
           exclude: solidPkgsConfig.optimizeDeps.exclude,