fix: add react-dom support.

docs/preview-root-lifecycle.md

@@ -0,0 +1,79 @@
+# knighted-preview-root lifecycle
+
+This document explains why `knighted-preview-root` exists, when it is present in preview, and when it is removed.
+
+## What is it
+
+`knighted-preview-root` is a custom host element created inside the preview iframe document.
+
+It is the dedicated React mount container used by the preview runtime in React render mode.
+
+## Why it exists even with iframe isolation
+
+The iframe isolates user code from the outer develop application, but React still needs a stable mount point inside the iframe document itself.
+
+Using `knighted-preview-root` provides:
+
+- A deterministic mount target for `createRoot(...)`.
+- Clear ownership of framework-rendered content within the iframe body.
+- Predictable cleanup between render passes.
+- A clean separation between React mode behavior and DOM mode behavior.
+
+In short:
+
+- Iframe isolation answers where code runs.
+- `knighted-preview-root` answers where React owns the DOM in that isolated page.
+
+## When it is created
+
+`knighted-preview-root` is created only during a successful React-mode render pass:
+
+1. The runtime receives a render request with mode set to React.
+2. The entry module is imported successfully.
+3. `App` resolves to a callable component.
+4. React output is created successfully.
+5. A new `knighted-preview-root` element is appended to `document.body`.
+6. React mounts into that host via `createRoot(host)`.
+
+## When it is removed
+
+At the beginning of every render pass, the runtime removes existing preview roots and clears previous render state.
+
+That means old `knighted-preview-root` nodes are intentionally deleted before the next render attempt.
+
+If the next render attempt fails before host creation, no new `knighted-preview-root` will be visible for that pass.
+
+## React mode vs DOM mode
+
+React mode:
+
+- Creates `knighted-preview-root`.
+- Mounts React output into that host.
+
+DOM mode:
+
+- Does not create `knighted-preview-root`.
+- Appends DOM output directly to the iframe `body`.
+
+So it is expected to sometimes not see `knighted-preview-root` when:
+
+- The current render mode is DOM.
+- The React render failed before host creation.
+- You inspect after cleanup but before a successful remount.
+
+## Portals and notification behavior
+
+In React mode, a portal target such as `document.body` points to the iframe body, not the outer develop UI document.
+
+This is expected and is part of preview encapsulation.
+
+`knighted-preview-root` does not change portal target semantics; it only defines the primary React mount host.
+
+## Quick troubleshooting checklist
+
+If `knighted-preview-root` is missing when you expected it:
+
+1. Confirm render mode is React.
+2. Confirm the latest pass did not fail before mount.
+3. Confirm you are inspecting the iframe document, not the parent document.
+4. Confirm auto-render actually scheduled a new render pass for the tab you edited.

src/bootstrap.js

@@ -12,6 +12,7 @@ const preloadImportKeys = [
   'jsxTransform',
   'jsxReact',
   'react',
+  'reactDom',
   'reactDomClient',
   'idb',
 ]

src/modules/cdn.js

@@ -69,6 +69,11 @@ export const cdnImportSpecs = {
     esm: 'react@19.2.4',
     jspmGa: 'npm:react@19.2.4/index.js',
   },
+  reactDom: {
+    importMap: 'react-dom',
+    esm: 'react-dom@19.2.4',
+    jspmGa: 'npm:react-dom@19.2.4/index.js',
+  },
   reactDomClient: {
     importMap: 'react-dom/client',
     esm: 'react-dom@19.2.4/client',

src/modules/preview-runtime/virtual-workspace-modules.js

@@ -423,6 +423,7 @@ const toStyleModuleDataUrl = ({ moduleKey, styleModuleExports }) => {
 
 const runtimeSpecifierRewrites = runtimeSpecifiers => ({
   react: runtimeSpecifiers.react,
+  'react-dom': runtimeSpecifiers.reactDom,
   'react-dom/client': runtimeSpecifiers.reactDomClient,
   '@knighted/jsx/dom': runtimeSpecifiers.jsxDom,
   '@knighted/jsx/react': runtimeSpecifiers.jsxReact,

src/modules/preview/render-runtime.js

@@ -701,6 +701,7 @@ export const createRenderRuntimeController = ({
     jsxDom: getRuntimeSpecifier('jsxDom'),
     jsxReact: getRuntimeSpecifier('jsxReact'),
     react: getRuntimeSpecifier('react'),
+    reactDom: getRuntimeSpecifier('reactDom'),
     reactDomClient: getRuntimeSpecifier('reactDomClient'),
   })