fix: insert position-area mapping styles into each target's own root

README.md

@@ -258,6 +258,26 @@ following features:
       containing block. To work around this, the polyfill strips any non-`auto`
       inset from the target (setting `inset: auto`) and re-applies it as padding
       on the wrapper, so the wrapper continues to drive positioning.
+    - Moving the target into the wrapper disconnects and reconnects it. If the
+      target is a custom element, its `connectedCallback` therefore runs more
+      than once, and any setup that can only happen once must be guarded — for
+      example, calling `attachShadow()` a second time throws. This applies to
+      any custom element the polyfill positions with `position-area`, including
+      a host positioned by a `position-area` in its own `:host` rule:
+
+      ```js
+      class MyElement extends HTMLElement {
+        connectedCallback() {
+          if (this.shadowRoot) return;
+          this.attachShadow({ mode: 'open' });
+          // ...
+        }
+      }
+      ```
+
+      Setting [`positionAreaContainingBlock`](#positionareacontainingblock) to
+      `false` (or `'auto'`, for targets that don't need the wrapper) avoids the
+      wrapper, and with it the reconnection.
   - When the wrapper is not added, styles that resolve against the containing
     block — percentage sizes, `auto` or percentage margins, percentage padding,
     or `stretch`/`anchor-center` self-alignment — will not match native

shadow-dom.html

@@ -97,6 +97,39 @@
           }
         }
         customElements.define('position-anchor-on-host', PositionAnchorOnHost);
+
+        // `position-area` in a `:host` rule positions the shadow *host*, which
+        // lives in the outer tree rather than in the shadow root the rule came
+        // from. The styles the polyfill generates to map the computed insets
+        // onto the target have to be inserted into that outer tree to match it.
+        const positionAreaOnHostSheet = new CSSStyleSheet();
+        positionAreaOnHostSheet.replaceSync(`
+          :host {
+            --element-color: var(--target, var(--outer-anchored));
+            background: var(--element-color);
+            border: thin solid var(--border);
+            border-radius: var(--radius-1);
+            color: white;
+            font-weight: bold;
+            padding: 0.5em;
+            white-space: nowrap;
+            position: absolute;
+            position-area: top;
+          }
+        `);
+
+        class PositionAreaOnHost extends HTMLElement {
+          connectedCallback() {
+            // Moving the host into the `position-area` wrapper disconnects and
+            // reconnects it, so this runs more than once.
+            if (this.shadowRoot) return;
+
+            this.attachShadow({ mode: 'open' });
+            this.shadowRoot.adoptedStyleSheets = [positionAreaOnHostSheet];
+            this.shadowRoot.innerHTML = '<slot></slot>';
+          }
+        }
+        customElements.define('position-area-on-host', PositionAreaOnHost);
       }
 
       const btn = document.getElementById('apply-polyfill');
@@ -478,6 +511,70 @@ <h2>
 }
 customElements.define("position-anchor-on-host", PositionAnchorOnHost);
 &lt;/script&gt;
+</code></pre>
+    </section>
+    <section id="position-area-on-host" class="demo-item">
+      <h2>
+        <a href="#position-area-on-host" aria-hidden="true">🔗</a>
+        Works when a custom element host has <code>position-area</code>
+      </h2>
+      <div style="position: relative" class="demo-elements">
+        <div
+          class="anchor"
+          style="
+            anchor-name: --position-area-on-host;
+            margin-block-start: calc(1lh + 1rem);
+          "
+        >
+          Anchor
+        </div>
+        <position-area-on-host style="position-anchor: --position-area-on-host"
+          >Target</position-area-on-host
+        >
+      </div>
+      <div class="note">
+        <p>With polyfill applied: Target sits directly above the Anchor.</p>
+        <p>
+          The <code>position-area</code> is declared in a
+          <code>:host</code> rule, so the element it positions is the host
+          (<code>&lt;position-area-on-host&gt;</code>), which lives in the outer
+          tree rather than in the shadow root the rule came from. The styles the
+          polyfill generates to map the computed insets onto the target are
+          inserted into the host's own tree; a <code>&lt;style&gt;</code> inside
+          the shadow root would never match the host.
+        </p>
+      </div>
+
+      <pre><code class="language-html"
+>&lt;div class="anchor" style="anchor-name: --position-area-on-host"&gt;Anchor&lt;/div&gt;
+&lt;position-area-on-host style="position-anchor: --position-area-on-host"&gt;Target&lt;/position-area-on-host&gt;
+&lt;script&gt;
+&lt;!-- Load the shadow entrypoint before defining custom elements,
+so the replaceSync and adoptedStyleSheets patches are installed
+before any connectedCallback runs. --&gt;
+import { patchAndPolyfillConstructedStylesheets } from '@oddbird/css-anchor-positioning/fn';
+patchAndPolyfillConstructedStylesheets();
+
+class PositionAreaOnHost extends HTMLElement {
+  connectedCallback() {
+    // Moving the host into the position-area wrapper reconnects it.
+    if (this.shadowRoot) return;
+
+    this.attachShadow({ mode: "open" });
+
+    const sheet = new CSSStyleSheet();
+    sheet.replaceSync(`
+      :host {
+        position: absolute;
+        position-area: top;
+      }
+    `);
+    this.shadowRoot.adoptedStyleSheets = [sheet];
+    this.shadowRoot.innerHTML = "&lt;slot&gt;&lt;/slot&gt;";
+  }
+}
+customElements.define("position-area-on-host", PositionAreaOnHost);
+&lt;/script&gt;
 </code></pre>
     </section>
     <section id="sponsor">

src/cascade.ts

@@ -113,6 +113,10 @@ export function registerShiftedProperties(
       .join('\n  ');
     for (const root of roots) {
       const container = getRootStyleContainer(root);
+      // A detached root has no container whose styles would reach it.
+      if (!container) {
+        continue;
+      }
       // Inject the reset once per container (a shadow root, or a document head
       // shared by several light-DOM roots). Dedupe against the live DOM: scope
       // the query to our own generated styles via the marker attribute, then

src/dom.ts

@@ -101,8 +101,9 @@ function createFakePseudoElement(
   // `content` rule (which sizes the fake pseudo-element) and the `display: none`
   // rule (which hides the real pseudo-element) would both be ignored when
   // `element` lives in a shadow tree. The fake pseudo-element is inserted into
-  // `element` below, so it shares this same root.
-  getRootStyleContainer(element).append(sheet);
+  // `element` below, so it shares this same root. A detached element has no
+  // container — and no layout to measure — so there is nothing to append to.
+  getRootStyleContainer(element)?.append(sheet);
 
   const insertionPoint =
     pseudoElementPart === '::before' ? 'afterbegin' : 'beforeend';

src/parse.ts

@@ -48,7 +48,9 @@ import {
 import {
   type DeclarationWithValue,
   generateCSS,
+  type GeneratedStyles,
   getAST,
+  getRootStyleContainer,
   getSelectors,
   isAnchorFunction,
   type StyleData,
@@ -809,14 +811,10 @@ export async function parseCSS(
     }
   }
 
-  // Create a new stylesheet for the position-area mapping styles
-  const positionAreaMappingStyleElement: StyleData = {
-    el: document.createElement('link'),
-    changed: false,
-    created: true,
-    css: '',
-  };
-  styleData.push(positionAreaMappingStyleElement);
+  // Collect the position-area mapping styles the polyfill generates. These are
+  // returned rather than added to `styleData`: they are polyfill output, not
+  // author styles to be rewritten in place.
+  const positionAreaStyles: GeneratedStyles = new Map();
 
   // We loop through each selector that has been used to apply a position-area
   // declaration, and find all elements that match the selector. The same
@@ -862,11 +860,19 @@ export async function parseCSS(
         const activeStyles = needsWrapper
           ? activeWrapperStyles
           : activeTargetStyles;
-        positionAreaMappingStyleElement.css += activeStyles(
-          targetData.targetUUID,
-          positionData.selectorUUID,
-        );
-        positionAreaMappingStyleElement.changed = true;
+        // These rules match the target (or the wrapper inserted next to it), so
+        // they belong in the target's own tree. That is not necessarily one of
+        // the roots being polyfilled: a `position-area` in a `:host` rule
+        // targets the shadow host, which lives outside the shadow root the
+        // declaration came from.
+        const container = getRootStyleContainer(targetEl);
+        if (container) {
+          positionAreaStyles.set(
+            container,
+            (positionAreaStyles.get(container) ?? '') +
+              activeStyles(targetData.targetUUID, positionData.selectorUUID),
+          );
+        }
         // Populate new data for each anchor/target combo
         validPositions[targetSel] = {
           ...validPositions[targetSel],
@@ -883,5 +889,10 @@ export async function parseCSS(
     }
   }
 
-  return { rules: validPositions, inlineStyles, anchorScopes };
+  return {
+    rules: validPositions,
+    inlineStyles,
+    anchorScopes,
+    positionAreaStyles,
+  };
 }

src/polyfill.ts

@@ -31,8 +31,9 @@ import {
   isInsetProp,
   type SizingProperty,
 } from './syntax.js';
-import { transformCSS } from './transform.js';
+import { insertGeneratedStyles, transformCSS } from './transform.js';
 import {
+  type GeneratedStyles,
   reportParseErrorsOnFailure,
   resetParseErrors,
   strategyForElement,
@@ -766,6 +767,7 @@ export async function polyfill(
   // eslint-disable-next-line no-useless-assignment
   let rules: AnchorPositions = {};
   let inlineStyles: Map<HTMLElement, Record<string, string>> | undefined;
+  let positionAreaStyles: GeneratedStyles;
 
   // Reset the CSS parse errors in case the polyfill is run multiple times, and
   // at the beginning in case a previous run failed.
@@ -784,6 +786,7 @@ export async function polyfill(
     const parsedCSS = await parseCSS(styleData, options);
     rules = parsedCSS.rules;
     inlineStyles = parsedCSS.inlineStyles;
+    positionAreaStyles = parsedCSS.positionAreaStyles;
   } catch (error) {
     reportParseErrorsOnFailure();
     throw error;
@@ -792,6 +795,7 @@ export async function polyfill(
   if (Object.values(rules).length) {
     // update source code
     transformCSS(styleData, inlineStyles, options.roots);
+    insertGeneratedStyles(positionAreaStyles);
 
     // calculate position values
     await position(rules, options.useAnimationFrame);

src/transform.ts

@@ -1,7 +1,7 @@
 import { POLYFILLED_STYLE_ATTRIBUTE } from './cascade.js';
 import type { AnchorPositioningRoot } from './polyfill.js';
 import {
-  getRootStyleContainer,
+  type GeneratedStyles,
   type StyleData,
   writeAdoptedStylesheet,
 } from './utils.js';
@@ -33,7 +33,7 @@ export function transformCSS(
   roots?: AnchorPositioningRoot[],
 ) {
   const updatedStyleData: StyleData[] = [];
-  for (const { el, css, changed, created = false, sheet } of styleData) {
+  for (const { el, css, changed, sheet } of styleData) {
     const updatedObject: StyleData = { el, css, changed: false, sheet };
     if (changed) {
       if (sheet) {
@@ -66,27 +66,8 @@ export function transformCSS(
         if (el.hasAttribute('href')) {
           styleEl.setAttribute('data-original-href', el.getAttribute('href')!);
         }
-        if (!created) {
-          // This is an existing stylesheet, so we replace it.
-          el.insertAdjacentElement('beforebegin', styleEl);
-          el.remove();
-        } else {
-          styleEl.setAttribute(POLYFILLED_STYLE_ATTRIBUTE, 'true');
-          // This is a new stylesheet (the position-area mapping styles). Its
-          // rules target wrapper elements that live inside the roots being
-          // polyfilled, so it must be inserted into each of those roots: a
-          // `<style>` in `document.head` does not apply inside a shadow root.
-          const containers = new Set(
-            (roots?.length ? roots : [document]).map(getRootStyleContainer),
-          );
-          for (const container of containers) {
-            // If there are multiple roots, clone the element for each root
-            const node = styleEl.isConnected
-              ? styleEl
-              : styleEl.cloneNode(true);
-            container.append(node);
-          }
-        }
+        el.insertAdjacentElement('beforebegin', styleEl);
+        el.remove();
         updatedObject.el = styleEl;
       } else if (el?.hasAttribute('data-has-inline-styles')) {
         // Handle inline styles
@@ -131,3 +112,17 @@ export function transformCSS(
   }
   return updatedStyleData;
 }
+
+/**
+ * Inserts styles the polyfill generated itself (the position-area mapping
+ * styles) into the container recorded for each block of rules.
+ */
+export function insertGeneratedStyles(styles: GeneratedStyles) {
+  for (const [container, css] of styles) {
+    if (!css) continue;
+    const styleEl = document.createElement('style');
+    styleEl.setAttribute(POLYFILLED_STYLE_ATTRIBUTE, 'true');
+    styleEl.textContent = css;
+    container.append(styleEl);
+  }
+}

src/utils.ts

@@ -69,12 +69,26 @@ export interface StyleData {
   css: string;
   url?: URL;
   changed?: boolean;
-  created?: boolean; // Whether the element is created by the polyfill
   // The constructed stylesheet this data came from, when the styles were
   // adopted via `adoptedStyleSheets` rather than a `<style>`/`<link>` element.
   sheet?: CSSStyleSheet;
 }
 
+// The node a polyfill-generated `<style>` is appended to, so its rules apply
+// within one tree. See `getRootStyleContainer`.
+export type StyleContainer = ShadowRoot | HTMLHeadElement;
+
+/**
+ * Styles the polyfill generates itself, rather than author styles it rewrites,
+ * keyed by the container each block of rules is inserted into.
+ *
+ * A `<style>` only applies within its own tree, so rules are grouped by the
+ * tree holding the elements they match, and each tree gets only its own rules.
+ * Those trees are not always the roots being polyfilled: a `position-area` in a
+ * `:host` rule targets the shadow host, which sits in the *outer* tree.
+ */
+export type GeneratedStyles = Map<StyleContainer, string>;
+
 // Reference to the native `CSSStyleSheet.prototype.replaceSync` so that the
 // polyfill can write transformed CSS back into a constructed stylesheet without
 // re-triggering the patched version (which would re-capture the text). In
@@ -174,13 +188,18 @@ export function writeAdoptedStylesheet(
 // for a given root, so its rules apply within that root. Styles in
 // `document.head` do not pierce into a shadow root, so styles for a shadow root
 // (or an element inside one) must be appended there instead.
+//
+// Returns `null` for an element in a detached tree: no stylesheet applies to it
+// and it isn't rendered, so there is no container whose rules could reach it.
 export function getRootStyleContainer(
   root: AnchorPositioningRoot,
-): ShadowRoot | HTMLHeadElement {
+): StyleContainer | null {
   if (root instanceof ShadowRoot) return root;
   if (root instanceof Document) return root.head;
   const rootNode = root.getRootNode();
-  return rootNode instanceof ShadowRoot ? rootNode : document.head;
+  if (rootNode instanceof ShadowRoot) return rootNode;
+  if (rootNode instanceof Document) return rootNode.head;
+  return null;
 }
 
 export const POSITION_ANCHOR_PROPERTY = `--position-anchor-${INSTANCE_UUID}`;