fix: address seventh round of PR review feedback

- Fix MIGRATION.md: correct old API names in Removed column
  (parseRepeatStartMarker not parseRepeatViewStartMarker,
   parseElementBoundaryStartMarker(content) not (node))
- Upgrade targetContentBinding errors from plain Error to
  HydrationTargetElementError with factories and node context
- Fix scanStop boundary: use previousSibling of boundary.first so the
  boundary node itself is still eligible for matching as a start marker
- Remove unused isShadowRoot helper function

Co-authored-by: Copilot <223556219+Copilot@users.noreply.github.com>

Author: janechu

packages/fast-element/MIGRATION.md

@@ -71,10 +71,10 @@ The hydration markers embedded in SSR output have been simplified from verbose,
 | `HydrationMarkup.isContentBindingStartMarker(data)` | `HydrationMarkup.isContentBindingStartMarker(data)` (unchanged signature, new implementation) |
 | `HydrationMarkup.isContentBindingEndMarker(data)` | `HydrationMarkup.isContentBindingEndMarker(data)` (unchanged signature, new implementation) |
 | `HydrationMarkup.parseAttributeBinding(element)` | `HydrationMarkup.parseAttributeBindingCount(element)` |
-| `HydrationMarkup.parseRepeatViewStartMarker(data)` | `HydrationMarkup.isRepeatViewStartMarker(data)` |
-| `HydrationMarkup.parseRepeatViewEndMarker(data)` | `HydrationMarkup.isRepeatViewEndMarker(data)` |
-| `HydrationMarkup.parseElementBoundaryStartMarker(node)` | `HydrationMarkup.isElementBoundaryStartMarker(node)` |
-| `HydrationMarkup.parseElementBoundaryEndMarker(node)` | `HydrationMarkup.isElementBoundaryEndMarker(node)` |
+| `HydrationMarkup.parseRepeatStartMarker(data)` | `HydrationMarkup.isRepeatViewStartMarker(data)` |
+| `HydrationMarkup.parseRepeatEndMarker(data)` | `HydrationMarkup.isRepeatViewEndMarker(data)` |
+| `HydrationMarkup.parseElementBoundaryStartMarker(content)` | `HydrationMarkup.isElementBoundaryStartMarker(node)` |
+| `HydrationMarkup.parseElementBoundaryEndMarker(content)` | `HydrationMarkup.isElementBoundaryEndMarker(node)` |
 
 ### Impact
 

packages/fast-element/src/hydration/target-builder.ts

@@ -74,10 +74,6 @@ export function createRangeForNodes(first: Node, last: Node): Range {
     return range;
 }
 
-function isShadowRoot(node: Node): node is ShadowRoot {
-    return node instanceof DocumentFragment && "mode" in node;
-}
-
 /**
  * Maps compiled ViewBehaviorFactory IDs to their corresponding DOM nodes in the
  * server-rendered shadow root. Uses a TreeWalker to scan the existing DOM between
@@ -179,6 +175,7 @@ export function buildViewBindingTargets(
                         node as Comment,
                         walker,
                         factory,
+                        factories,
                         targets,
                         boundaries,
                     );
@@ -198,6 +195,7 @@ function targetContentBinding(
     node: Comment,
     walker: TreeWalker,
     factory: CompiledViewBehaviorFactory,
+    factories: CompiledViewBehaviorFactory[],
     targets: ViewBehaviorTargets,
     boundaries: ViewBehaviorBoundaries,
 ) {
@@ -206,11 +204,12 @@ function targetContentBinding(
     node.data = "";
 
     if (current === null) {
-        const root = node.getRootNode();
-        throw new Error(
-            `Error hydrating Comment node inside "${
-                isShadowRoot(root) ? root.host.nodeName : root.nodeName
+        throw new HydrationTargetElementError(
+            `Error hydrating content binding inside "${
+                (node.getRootNode() as ShadowRoot).host?.nodeName ?? "unknown"
             }": no sibling found after content binding start marker.`,
+            factories,
+            node,
         );
     }
 
@@ -232,11 +231,12 @@ function targetContentBinding(
     }
 
     if (current === null) {
-        const root = node.getRootNode();
-        throw new Error(
-            `Error hydrating Comment node inside "${
-                isShadowRoot(root) ? root.host.nodeName : root.nodeName
-            }".`,
+        throw new HydrationTargetElementError(
+            `Error hydrating content binding inside "${
+                (node.getRootNode() as ShadowRoot).host?.nodeName ?? "unknown"
+            }": missing fe:/b end marker.`,
+            factories,
+            node,
         );
     }
 

packages/fast-element/src/templating/repeat.ts

@@ -415,10 +415,14 @@ export class RepeatBehavior<TSource = any> implements ViewBehavior, Subscriber {
         // Determine the scan boundary for this repeat directive.
         // Use the content binding boundaries (from the parent's fe:b/fe:/b
         // markers) to avoid scanning into sibling repeat blocks.
+        // Set scanStop to the node BEFORE the boundary's first node so the
+        // boundary node itself is still eligible for matching as a start marker.
         const boundaries =
             isHydratable(this.controller) &&
             this.controller.bindingViewBoundaries[this.directive.targetNodeId];
-        const scanStop: Node | null = boundaries ? boundaries.first : null;
+        const scanStop: Node | null = boundaries
+            ? (boundaries.first.previousSibling ?? null)
+            : null;
 
         let current = this.location.previousSibling;
         let itemIndex = itemCount - 1; // items render in order; walk backward