fix: remove repeat overflow/underflow checks and scanStop boundary

The overflow check (scan for extra fe:/r markers after hydration) and
the scanStop boundary (bindingViewBoundaries-based scan limit) both
caused false positives with nested repeats. Data-free markers make it
impossible to distinguish fe:/r from this repeat vs a nested repeat at
the same DOM depth. The underflow check also misfired when nested
repeat items were counted instead of this repeat's items.

Remove all three mechanisms. The balanced depth counting in the backward
scan correctly handles nesting without them. This matches the original
base branch behavior which also did not have overflow/underflow checks.

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

Author: janechu

packages/fast-element/SIZES.md

@@ -4,7 +4,7 @@ Bundle sizes for `@microsoft/fast-element` exports.
 
 | Export | Minified | Gzip | Brotli |
 |--------|----------|------|--------|
-| CDN Rollup Bundle | 66.24 KB | 19.46 KB | 17.46 KB |
+| CDN Rollup Bundle | 64.96 KB | 19.28 KB | 17.28 KB |
 | FASTElement | 23.92 KB | 7.48 KB | 6.76 KB |
 | Updates | 3.68 KB | 1.51 KB | 1.28 KB |
 | Observable | 8.20 KB | 3.01 KB | 2.69 KB |
@@ -17,4 +17,4 @@ Bundle sizes for `@microsoft/fast-element` exports.
 | volatile | 8.29 KB | 3.04 KB | 2.71 KB |
 | when | 2.40 KB | 979 B | 787 B |
 | html | 27.37 KB | 8.95 KB | 8.03 KB |
-| repeat | 32.22 KB | 10.06 KB | 9.05 KB |
+| repeat | 30.96 KB | 9.86 KB | 8.87 KB |

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

@@ -407,27 +407,12 @@ export class RepeatBehavior<TSource = any> implements ViewBehavior, Subscriber {
         }
 
         const itemCount = this.items.length;
-        let serializer: XMLSerializer | null = null;
-        const getSerializer = (): XMLSerializer =>
-            serializer ?? (serializer = new XMLSerializer());
         this.views = Array.from({ length: itemCount }, () => null as any);
 
-        // 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.previousSibling ?? null)
-            : null;
-
         let current = this.location.previousSibling;
         let itemIndex = itemCount - 1; // items render in order; walk backward
 
-        while (current !== null && current !== scanStop && itemIndex >= 0) {
+        while (current !== null && itemIndex >= 0) {
             if (!isCommentNode(current) || current.data !== "fe:/r") {
                 current = current.previousSibling;
                 continue;
@@ -437,26 +422,17 @@ export class RepeatBehavior<TSource = any> implements ViewBehavior, Subscriber {
             current.data = "";
             const end = current.previousSibling;
             if (!end) {
-                throw new HydrationRepeatError(
+                throw new Error(
                     `Error when hydrating inside "${
                         (this.location.getRootNode() as ShadowRoot).host.nodeName
                     }": end should never be null.`,
-                    {
-                        index: itemIndex,
-                        hydrationStage: "hydrateViews",
-                        itemsLength: itemCount,
-                        viewsState: this.views.map(v => (v ? "hydrated" : "empty")),
-                        rootNodeContent: getSerializer().serializeToString(
-                            this.location.getRootNode() as any,
-                        ),
-                    },
                 );
             }
 
             // Find matching start marker via balanced counting
             let start: Node | null = end;
             let depth = 0;
-            while (start !== null && start !== scanStop) {
+            while (start !== null) {
                 if (isCommentNode(start)) {
                     if (start.data === "fe:/r") {
                         depth++;
@@ -485,67 +461,14 @@ export class RepeatBehavior<TSource = any> implements ViewBehavior, Subscriber {
                 }
                 start = start.previousSibling;
             }
-            if (!start || start === scanStop) {
-                throw new HydrationRepeatError(
+            if (!start) {
+                throw new Error(
                     `Error when hydrating inside "${
                         (this.location.getRootNode() as ShadowRoot).host.nodeName
-                    }": start should never be null.`,
-                    {
-                        index: itemIndex,
-                        hydrationStage: "hydrateViews",
-                        itemsLength: itemCount,
-                        viewsState: this.views.map(v => (v ? "hydrated" : "empty")),
-                        rootNodeContent: getSerializer().serializeToString(
-                            this.location.getRootNode() as any,
-                        ),
-                    },
+                    }": repeat start marker not found.`,
                 );
             }
         }
-
-        if (itemIndex >= 0) {
-            throw new HydrationRepeatError(
-                `Error when hydrating inside "${
-                    (this.location.getRootNode() as ShadowRoot).host.nodeName
-                }": expected ${itemCount} repeat items but only found ${itemCount - itemIndex - 1}.`,
-                {
-                    index: itemIndex,
-                    hydrationStage: "hydrateViews",
-                    itemsLength: itemCount,
-                    viewsState: this.views.map(v => (v ? "hydrated" : "empty")),
-                    rootNodeContent: getSerializer().serializeToString(
-                        this.location.getRootNode() as any,
-                    ),
-                },
-            );
-        }
-
-        // Overflow check: detect extra repeat markers beyond items.length,
-        // bounded to this directive's scan range.
-        // Skip when itemCount is 0 — an empty repeat has no markers to overflow,
-        // and the scan would cross into unrelated sibling content.
-        if (itemCount > 0 && current !== null && current !== scanStop) {
-            let remaining: Node | null = current;
-            while (remaining !== null && remaining !== scanStop) {
-                if (isCommentNode(remaining) && remaining.data === "fe:/r") {
-                    throw new HydrationRepeatError(
-                        `Error when hydrating inside "${
-                            (this.location.getRootNode() as ShadowRoot).host.nodeName
-                        }": found more repeat items in the DOM than expected ${itemCount}.`,
-                        {
-                            index: -1,
-                            hydrationStage: "hydrateViews",
-                            itemsLength: itemCount,
-                            viewsState: this.views.map(v => (v ? "hydrated" : "empty")),
-                            rootNodeContent: getSerializer().serializeToString(
-                                this.location.getRootNode() as any,
-                            ),
-                        },
-                    );
-                }
-                remaining = remaining.previousSibling;
-            }
-        }
     }
 }
 

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

@@ -19,7 +19,7 @@ Bundle sizes for `@microsoft/fast-element` exports.
 
 | Export | Minified | Gzip | Brotli |
 |--------|----------|------|--------|
-| CDN Rollup Bundle | 66.24 KB | 19.46 KB | 17.46 KB |
+| CDN Rollup Bundle | 64.96 KB | 19.28 KB | 17.28 KB |
 | FASTElement | 23.92 KB | 7.48 KB | 6.76 KB |
 | Updates | 3.68 KB | 1.51 KB | 1.28 KB |
 | Observable | 8.20 KB | 3.01 KB | 2.69 KB |
@@ -32,4 +32,4 @@ Bundle sizes for `@microsoft/fast-element` exports.
 | volatile | 8.29 KB | 3.04 KB | 2.71 KB |
 | when | 2.40 KB | 979 B | 787 B |
 | html | 27.37 KB | 8.95 KB | 8.03 KB |
-| repeat | 32.22 KB | 10.06 KB | 9.05 KB |
+| repeat | 30.96 KB | 9.86 KB | 8.87 KB |