fix: remove shared kernel (#7487)

# Pull Request

## 📖 Description

This removes the shared kernel now that multiple versions are no longer supported.

## ✅ Checklist

### General

- [x] I have included a change request file using `$ npm run change`
- [ ] I have added tests for my changes.
- [ ] I have tested my changes.
- [x] I have updated the project documentation to reflect my changes.
- [x] I have read the [CONTRIBUTING](https://github.com/microsoft/fast/blob/main/CONTRIBUTING.md) documentation and followed the [standards](https://github.com/microsoft/fast/blob/main/CODE_OF_CONDUCT.md#our-standards) for this project.

Author: janechu

change/@microsoft-fast-element-f5c8274a-52ea-4f04-90e2-f51ca033abc1.json

@@ -0,0 +1,7 @@
+{
+  "type": "major",
+  "comment": "fix: remove the fast-kernel multi-kernel modes. Breaking change: FAST no longer supports configuring isolated or version-scoped kernels via the script attribute.",
+  "packageName": "@microsoft/fast-element",
+  "email": "7559015+janechu@users.noreply.github.com",
+  "dependentChangeType": "none"
+}

packages/fast-element/DESIGN.md

@@ -69,13 +69,10 @@ engines must install their own `globalThis` polyfill before FAST loads.
 - `FAST.warn(code, values)` / `FAST.error(code, values)` – structured diagnostic messages
 - `FAST.addMessages(dict)` – registers human-readable debug messages (imported by `src/debug.ts`)
 
-The `KernelServiceId` object controls which numeric/string keys are used for shared services. Three modes are supported via a `fast-kernel` attribute on the current `<script>` tag:
-
-| Mode | Behaviour |
-|---|---|
-| `share` | Use the shared numeric kernel service IDs |
-| `share-v2` | Use the alternate numeric kernel service IDs |
-| *(default)* | Fully isolated instance with a random postfix |
+The `KernelServiceId` enum provides the fixed numeric keys used for shared
+services on the `FAST` global. These stable IDs let FAST instances on the same
+page reuse the update queue, observable system, context event, and element
+registry.
 
 ---
 

packages/fast-element/MIGRATION.md

@@ -8,10 +8,17 @@
 |---|---|
 | `FAST.versions` | No replacement. Multiple FAST versions on the same page are unsupported in v3. |
 
+### Removed configuration
+
+| Removed | Replacement |
+|---|---|
+| `fast-kernel="share"` / `fast-kernel="share-v2"` / `fast-kernel="isolate"` | No replacement. FAST now uses a single shared kernel contract, and multiple FAST versions on the same page are unsupported. |
+
 ### Migration steps
 
 1. Remove any runtime checks that read `FAST.versions`.
 2. Fix duplicate FAST installs in your bundler or dependency graph instead of relying on version tracking at runtime.
+3. Remove any `fast-kernel` script attributes. They no longer affect FAST initialization.
 
 ## Native `globalThis` requirement (v2 → v3)
 

packages/fast-element/SIZES.md

@@ -4,17 +4,17 @@ Bundle sizes for `@microsoft/fast-element` exports.
 
 | Export | Minified | Gzip | Brotli |
 |--------|----------|------|--------|
-| CDN Rollup Bundle | 63.06 KB | 18.80 KB | 16.88 KB |
-| FASTElement | 23.30 KB | 7.32 KB | 6.60 KB |
-| Updates | 3.45 KB | 1.43 KB | 1.21 KB |
-| Observable | 7.96 KB | 2.94 KB | 2.62 KB |
-| observable | 8.00 KB | 2.95 KB | 2.62 KB |
-| attr | 3.39 KB | 1.37 KB | 1.13 KB |
-| children | 5.99 KB | 2.29 KB | 2.01 KB |
-| css | 4.47 KB | 1.78 KB | 1.54 KB |
-| ref | 4.96 KB | 1.96 KB | 1.71 KB |
-| slotted | 5.78 KB | 2.22 KB | 1.95 KB |
-| volatile | 8.05 KB | 2.97 KB | 2.63 KB |
-| when | 2.40 KB | 978 B | 786 B |
-| html | 27.14 KB | 8.88 KB | 7.96 KB |
-| repeat | 30.79 KB | 9.81 KB | 8.82 KB |
+| CDN Rollup Bundle | 62.60 KB | 18.64 KB | 16.77 KB |
+| FASTElement | 22.88 KB | 7.14 KB | 6.46 KB |
+| Updates | 3.03 KB | 1.25 KB | 1.06 KB |
+| Observable | 7.55 KB | 2.76 KB | 2.48 KB |
+| observable | 7.58 KB | 2.77 KB | 2.50 KB |
+| attr | 2.98 KB | 1.18 KB | 1011 B |
+| children | 5.58 KB | 2.15 KB | 1.90 KB |
+| css | 4.06 KB | 1.60 KB | 1.40 KB |
+| ref | 4.55 KB | 1.81 KB | 1.60 KB |
+| slotted | 5.36 KB | 2.08 KB | 1.83 KB |
+| volatile | 7.64 KB | 2.79 KB | 2.51 KB |
+| when | 1.99 KB | 770 B | 625 B |
+| html | 26.71 KB | 8.74 KB | 7.84 KB |
+| repeat | 30.37 KB | 9.67 KB | 8.70 KB |

packages/fast-element/src/interfaces.ts

@@ -103,66 +103,17 @@ export interface FASTGlobal {
     addMessages(messages: Record<number, string>): void;
 }
 
-let kernelMode;
-const kernelAttr = "fast-kernel";
-
-try {
-    if (document.currentScript) {
-        kernelMode = document.currentScript.getAttribute(kernelAttr);
-    } else {
-        const scripts = document.getElementsByTagName("script");
-        const currentScript = scripts[scripts.length - 1];
-        kernelMode = currentScript.getAttribute(kernelAttr);
-    }
-} catch {
-    kernelMode = "isolate";
-}
-
 /**
- * Core services that can be shared across FAST instances.
+ * Core services shared across FAST instances.
  * @internal
  */
-type KernelServiceId = {
-    readonly updateQueue: string | number;
-    readonly observable: string | number;
-    readonly contextEvent: string | number;
-    readonly elementRegistry: string | number;
-};
-
-let KernelServiceId: KernelServiceId;
-
-switch (kernelMode) {
-    case "share": // share the kernel across major versions
-        KernelServiceId = Object.freeze({
-            updateQueue: 1,
-            observable: 2,
-            contextEvent: 3,
-            elementRegistry: 4,
-        });
-        break;
-    case "share-v2": // only share the kernel with other v2 instances
-        KernelServiceId = Object.freeze({
-            updateQueue: 1.2,
-            observable: 2.2,
-            contextEvent: 3.2,
-            elementRegistry: 4.2,
-        });
-        break;
-    default: {
-        // fully isolate the kernel from all other FAST instances
-        const postfix = `-${Math.random().toString(36).substring(2, 8)}`;
-        KernelServiceId = Object.freeze({
-            updateQueue: `1.2${postfix}`,
-            observable: `2.2${postfix}`,
-            contextEvent: `3.2${postfix}`,
-            elementRegistry: `4.2${postfix}`,
-        });
-        break;
-    }
+export const enum KernelServiceId {
+    updateQueue = 1,
+    observable = 2,
+    contextEvent = 3,
+    elementRegistry = 4,
 }
 
-export { KernelServiceId };
-
 /**
  * Warning and error messages.
  * @internal

sites/website/src/docs/3.x/resources/export-sizes.md

@@ -19,17 +19,17 @@ Bundle sizes for `@microsoft/fast-element` exports.
 
 | Export | Minified | Gzip | Brotli |
 |--------|----------|------|--------|
-| CDN Rollup Bundle | 63.06 KB | 18.80 KB | 16.88 KB |
-| FASTElement | 23.30 KB | 7.32 KB | 6.60 KB |
-| Updates | 3.45 KB | 1.43 KB | 1.21 KB |
-| Observable | 7.96 KB | 2.94 KB | 2.62 KB |
-| observable | 8.00 KB | 2.95 KB | 2.62 KB |
-| attr | 3.39 KB | 1.37 KB | 1.13 KB |
-| children | 5.99 KB | 2.29 KB | 2.01 KB |
-| css | 4.47 KB | 1.78 KB | 1.54 KB |
-| ref | 4.96 KB | 1.96 KB | 1.71 KB |
-| slotted | 5.78 KB | 2.22 KB | 1.95 KB |
-| volatile | 8.05 KB | 2.97 KB | 2.63 KB |
-| when | 2.40 KB | 978 B | 786 B |
-| html | 27.14 KB | 8.88 KB | 7.96 KB |
-| repeat | 30.79 KB | 9.81 KB | 8.82 KB |
+| CDN Rollup Bundle | 62.60 KB | 18.64 KB | 16.77 KB |
+| FASTElement | 22.88 KB | 7.14 KB | 6.46 KB |
+| Updates | 3.03 KB | 1.25 KB | 1.06 KB |
+| Observable | 7.55 KB | 2.76 KB | 2.48 KB |
+| observable | 7.58 KB | 2.77 KB | 2.50 KB |
+| attr | 2.98 KB | 1.18 KB | 1011 B |
+| children | 5.58 KB | 2.15 KB | 1.90 KB |
+| css | 4.06 KB | 1.60 KB | 1.40 KB |
+| ref | 4.55 KB | 1.81 KB | 1.60 KB |
+| slotted | 5.36 KB | 2.08 KB | 1.83 KB |
+| volatile | 7.64 KB | 2.79 KB | 2.51 KB |
+| when | 1.99 KB | 770 B | 625 B |
+| html | 26.71 KB | 8.74 KB | 7.84 KB |
+| repeat | 30.37 KB | 9.67 KB | 8.70 KB |