Tal500/legacy-builds revitalized 2026 bare merge

[gamesguru]

@cubic-dev-ai review this pull request

---

Summary by cubic

Add first-class legacy browser builds (IE11+) by auto-integrating @vitejs/plugin-legacy, emitting modern and legacy client bundles with polyfills and safe bootstrapping for nomodule browsers. The fork is published as @tg-svelte/kit and includes a legacy-friendly browserslist.

• New Features

  • Zero-config handling of Vite’s multi-output; correctly resolves modern/legacy entry files from the manifest.
  • Injects modern/legacy polyfills, preloads modern polyfills, and applies the Safari 10 nomodule fix.
  • Startup uses dynamic import detection with a fallback to legacy chunks; dev mode stays modern-only.

• Bug Fixes

  • Form enhance requests explicitly set credentials: 'same-origin' to align with whatwg-fetch defaults.
  • CSRF check falls back to Referer when Origin is missing in older browsers.
  • Data loader works without Response.body streams by falling back to line-based decoding.

^{Written for commit 731ead3. Summary will update on new commits.}

[cubic-dev-ai]

5 issues found across 503 files

Confidence score: 2/5

• SSR error handling is currently broken because packages/kit/src/runtime/server/page/render.js still pushes serialized.error into the hydrate array after removing the serialization logic, so clients never receive server errors.
• Streaming data loaders in legacy environments will break since packages/kit/src/runtime/client/client.js now emits NDJSON chunks without newline delimiters when falling back from Response, leaving parsers stuck.
• Documentation tweaks in documentation/legacy/PR_DRAFT.md (branch name and typo) and the stray JSDoc on packages/kit/src/runtime/server/page/render.js don’t block merging but should be cleaned up for clarity.
• Pay close attention to packages/kit/src/runtime/server/page/render.js, packages/kit/src/runtime/client/client.js, and documentation/legacy/PR_DRAFT.md – error propagation, NDJSON parsing, and docs accuracy need fixes.

Note: This PR contains a large number of files. cubic only reviews up to 75 files per PR, so some files may not have been reviewed.

Prompt for AI agents (all issues)

Check if these issues are valid — if so, understand the root cause of each and fix them.


<file name="documentation/legacy/PR_DRAFT.md">

<violation number="1" location="documentation/legacy/PR_DRAFT.md:10">
P3: Fix the typo by spelling “properties” correctly so the documentation is clear.</violation>

<violation number="2" location="documentation/legacy/PR_DRAFT.md:25">
P2: The branch creation instructions reference `origin/master`, but this repository’s default branch is `main`, so the command will fail. Update the command to track `origin/main`.</violation>
</file>

<file name="packages/kit/src/runtime/client/client.js">

<violation number="1" location="packages/kit/src/runtime/client/client.js:2930">
P2: The legacy Response fallback drops newline delimiters, so NDJSON chunks can no longer be parsed in environments without `Response.body`, breaking data loading there.</violation>
</file>

<file name="packages/kit/src/runtime/server/page/render.js">

<violation number="1" location="packages/kit/src/runtime/server/page/render.js:342">
P2: Stray JSDoc comment with `@param` annotations is attached to the `hydrate` array declaration, which doesn't take any parameters. This comment should be removed as it's misleading.</violation>

<violation number="2" location="packages/kit/src/runtime/server/page/render.js:373">
P0: Critical bug: The error serialization code was removed, but `serialized.error` is still used in the hydrate array. This means errors will never be properly passed to the client, breaking SSR error handling. The `if (error) { serialized.error = devalue.uneval(error); }` block needs to be restored.</violation>
</file>

_{Reply with feedback, questions, or to request a fix. Tag @cubic-dev-ai to re-run a review.}

[cubic-dev-ai]

P0: Critical bug: The error serialization code was removed, but serialized.error is still used in the hydrate array. This means errors will never be properly passed to the client, breaking SSR error handling. The if (error) { serialized.error = devalue.uneval(error); } block needs to be restored.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At packages/kit/src/runtime/server/page/render.js, line 373:

<comment>Critical bug: The error serialization code was removed, but `serialized.error` is still used in the hydrate array. This means errors will never be properly passed to the client, breaking SSR error handling. The `if (error) { serialized.error = devalue.uneval(error); }` block needs to be restored.</comment>

<file context>
@@ -328,6 +330,250 @@ export async function render_response({
+				if (input_list.length === 0) {
+					return codeBlocks.join(separator);
+				}
+				// otherwise
+
+				return legacy_support_and_export_init
</file context>

Suggested change

	// otherwise
	if (error) {
	serialized.error = devalue.uneval(error);
	}

[cubic-dev-ai]

P2: The branch creation instructions reference origin/master, but this repository’s default branch is main, so the command will fail. Update the command to track origin/main.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At documentation/legacy/PR_DRAFT.md, line 25:

<comment>The branch creation instructions reference `origin/master`, but this repository’s default branch is `main`, so the command will fail. Update the command to track `origin/main`.</comment>

<file context>
@@ -0,0 +1,36 @@
+
+1.  **Create a Clean Branch**:
+    ```bash
+    git checkout -b fix/legacy-plugin-support origin/master
+    ```
+
</file context>

Suggested change

	git checkout -b fix/legacy-plugin-support origin/master
	+ git checkout -b fix/legacy-plugin-support origin/main

[cubic-dev-ai]

P2: The legacy Response fallback drops newline delimiters, so NDJSON chunks can no longer be parsed in environments without Response.body, breaking data loading there.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At packages/kit/src/runtime/client/client.js, line 2930:

<comment>The legacy Response fallback drops newline delimiters, so NDJSON chunks can no longer be parsed in environments without `Response.body`, breaking data loading there.</comment>

<file context>
@@ -2895,12 +2895,58 @@ async function load_data(url, invalid) {
+						if (current_line >= text_lines.length) {
+							return { done: true, value: undefined };
+						} else {
+							return { done: false, value: text_lines[current_line++] };
+						}
+					}
</file context>

[cubic-dev-ai]

P2: Stray JSDoc comment with @param annotations is attached to the hydrate array declaration, which doesn't take any parameters. This comment should be removed as it's misleading.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At packages/kit/src/runtime/server/page/render.js, line 342:

<comment>Stray JSDoc comment with `@param` annotations is attached to the `hydrate` array declaration, which doesn't take any parameters. This comment should be removed as it's misleading.</comment>

<file context>
@@ -328,6 +330,250 @@ export async function render_response({
+
+		const modern_import_func_var_name = '__KIT_modern_import_func';
+
+		/**
+		 * Generate JS init code for the HTML entry page
+		 * @param {boolean} legacy_support_and_export_init
</file context>

[cubic-dev-ai]

P3: Fix the typo by spelling “properties” correctly so the documentation is clear.

Prompt for AI agents

Check if this issue is valid — if so, understand the root cause and fix it. At documentation/legacy/PR_DRAFT.md, line 10:

<comment>Fix the typo by spelling “properties” correctly so the documentation is clear.</comment>

<file context>
@@ -0,0 +1,36 @@
+This PR addresses build failures when using `@vitejs/plugin-legacy` with SvelteKit.
+
+### The Problem
+When `@vitejs/plugin-legacy` is used, Vite produces an array of output bundles (one for modern, one for legacy) instead of a single output object. SvelteKit's `writeBundle` hook previously assumed a single output, causing `TypeError`s when accessing properies like `bundle.output`.
+
+Additionally, the legacy plugin may rename/remap entry chunks. SvelteKit was strictly looking for `start.js` (or `entry.js`), resulting in "Entry not found" errors when the manifest contained `entry-legacy.js` or similar variations.
</file context>

Suggested change

	When `@vitejs/plugin-legacy` is used, Vite produces an array of output bundles (one for modern, one for legacy) instead of a single output object. SvelteKit's `writeBundle` hook previously assumed a single output, causing `TypeError`s when accessing properies like `bundle.output`.
	+When `@vitejs/plugin-legacy` is used, Vite produces an array of output bundles (one for modern, one for legacy) instead of a single output object. SvelteKit's `writeBundle` hook previously assumed a single output, causing `TypeError`s when accessing properties like `bundle.output`.