fix(virtual-core): preserve measurement keys and follow sliding windows

[tigerBeA]

🎯 Changes

Fixes #1270.
Fixes #1271.

Keep measurement keys consistent across data updates, then use those keys to follow appended output when older items are trimmed in the same update.

Both failures have standalone live reproductions using published packages:

Reproduction	Published core 3.17.9	Local candidate
Cached item identity, following disabled	8 rendered key/ID mismatches; reading anchor moves by -50 px	0 mismatches; reading anchor stays in place
Append + trim, initially at the end	50 px bottom gap	0 px bottom gap

Reproduction source and steps.

The implementation is split into two commits for review:

1. Preserve keys in lazy measurement builds. Single-lane cache slots initially hold the keys used to construct the layout, then hold VirtualItem objects after first access. Partial rebuilds retain the untouched prefix. Unread items no longer call a stable getItemKey against newer data, so old edge comparisons and reading anchors use the correct identities.
2. Follow append-and-trim updates. For non-growing updates, recognize an ordered, non-empty suffix of the old list followed by new keys. These non-growing updates are not followed when they are replacements, reorders, rotations, trim-only updates, or prepends. The existing count-growing classification is unchanged; this PR does not add key-continuity validation to that path. Following still requires the viewport to have been within scrollEndThreshold. When following wins, avoid eagerly assigning the reading-anchor offset: an equal-height window update can leave the DOM offset unchanged and emit no scroll event to correct that assignment.

No public options are added. One patch changeset covers @tanstack/virtual-core, and the followOnAppend documentation describes the retained-overlap requirement.

Cost

Keys use the existing lazy-cache slots rather than a parallel key array; VirtualItem objects remain lazy and the flat position/size buffer is reused. The overlap scan reads raw keys without materializing all old rows and runs only for end-pinned, non-growing append candidates. The tradeoff is storing keys during the layout loop and copying cache slots for partial rebuilds.

See the performance measurements and rerun instructions for comparison sources and recorded resize, memory, browser-update, and native-scroll results. At 10,000 rows, head-heavy rebuilds added about 12–16 µs per operation (8.5–11.5%); the ordinary-scroll samples showed no consistent slowdown. The baseline is an earlier local window-follow fix, not the unmodified release. Browser timings exclude paint.

Validation

• With the original core source and the new cache tests, all 3 cache regressions fail: unread keys resolve to the next item's key, and the reading anchor receives no adjustment.
• With only the first commit, 7 window-follow cases fail; restoring the second commit makes them pass.
• 60 targeted core tests pass, covering lazy keys, partial rebuilds, end anchoring, consecutive window updates, thresholds, disabled following, smooth following, count growth/reduction, and non-append controls. The growing-window compatibility case removes 1 item and appends 2; it passes against both the upstream base and this PR. No persistent skips were added.
• Chrome production-build checks confirm the live failures and the local candidate results above. Ordinary append still follows, and native scrolling away from the end retains the reading anchor on the next window update.
• Prettier checks pass for the changed TypeScript files and changeset.
• Before the latest test-only follow-up, NX_NO_CLOUD=true NX_DAEMON=false NX_BASE=origin/main NX_HEAD=HEAD CI=1 pnpm run test:pr --parallel=2 --skip-nx-cache passed for all 69 affected projects at c452d482, including 161 core tests, browser tests, type checks, lint, documentation/package checks, and builds. The follow-up adds only one test parameter; production source is unchanged, and the 60-test targeted run above covers the updated file. Runtime: Node 24.8.0 from .nvmrc, pnpm 11.9.0 from packageManager.

✅ Checklist

• I have followed the steps in the Contributing guide.
• I have tested this code locally with pnpm run test:pr.

🚀 Release Impact

• This change affects published code, and I have generated a patch changeset for @tanstack/virtual-core.
• This change is docs/CI/dev-only (no release).

Summary by CodeRabbit

• Bug Fixes

  • End-pinned virtualizers now continue following newly appended items when older items are trimmed in the same update.
  • Previously measured item keys remain stable when underlying data changes, preventing rows from unexpectedly changing identity.
  • Improved reading-anchor preservation across sliding-window updates, including smooth scrolling and updates without scroll events.

• Documentation

  • Clarified requirements and limitations for following appended items during sliding-window updates, including persistent keys, retained items, and cases that are not automatically followed.

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📥 Commits

Reviewing files that changed from the base of the PR and between 789f5c2 and e89ef7d.

📒 Files selected for processing (5)

• .changeset/follow-sliding-window.md
• docs/api/virtualizer.md
• packages/virtual-core/src/index.ts
• packages/virtual-core/src/lazy-measurements.ts
• packages/virtual-core/tests/index.test.ts

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📝 Walkthrough

Walkthrough

The virtualizer preserves keys in lazy single-lane measurements and recognizes valid append-with-trim updates for end following. Tests cover anchor preservation, follow behavior, thresholds, disabled states, and excluded update patterns.

Changes

Virtualizer sliding-window behavior

Layer / File(s)	Summary
Preserve measurement keys packages/virtual-core/src/lazy-measurements.ts, packages/virtual-core/src/index.ts, packages/virtual-core/tests/index.test.ts	Single-lane measurements store per-index keys with layout data. Lazy views materialize items from stored keys. Related measurement readers use the new structure.
Follow append-with-trim updates packages/virtual-core/src/index.ts, packages/virtual-core/tests/index.test.ts, docs/api/virtualizer.md, .changeset/follow-sliding-window.md	followOnAppend recognizes append-with-trim updates when key continuity conditions hold. Tests cover following, anchoring, thresholds, and excluded update patterns. Documentation and the changeset describe the behavior.

Priority: ➖ Normal — Impact reflects medium issue severity.

Estimated code review effort: 3 (Moderate) | ~20 minutes

Severity of issue fixed: Medium

Merge Risk: 🟡 Moderate · up to e89ef

This updates sliding-window key retention and end-follow behavior, but count-growing replacement or reorder updates may still force users to the end rather than preserve their reading position. Resolve this before merge.

Sequence Diagram(s)

sequenceDiagram
  participant DataUpdate
  participant Virtualizer
  participant MeasurementCache
  participant ScrollController
  DataUpdate->>Virtualizer: provide updated keyed items
  Virtualizer->>MeasurementCache: rebuild measurements and preserve keys
  MeasurementCache-->>Virtualizer: return keyed virtual items
  Virtualizer->>ScrollController: follow valid append-with-trim update

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Suggested reviewers: piecyk, 2wheeh

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 3 files. (2 skipped: 2 …	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Linked Issues check	✅ Passed	The implementation addresses both linked issues. It preserves measurement keys and reading anchors for same-count updates [#1270], and follows valid append-and-trim updates while excluding replacement…
Out of Scope Changes check	✅ Passed	The code, tests, documentation, and changeset changes support the stated objectives. No unrelated or out-of-scope changes are identified.
Title check	✅ Passed	The title clearly identifies the two main changes: preserving measurement keys and following sliding-window updates.
Description check	✅ Passed	The description is complete and follows the required template. It explains the changes, motivation, validation, checklist status, release impact, changeset, and documentation updates.

Full details: Docstring Coverage

Explanation

Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 3 functions across 3 files. (2 skipped: 2 unsupported.)

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Actionable comments posted: 1

🤖 Prompt for all review comments with AI agents

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Inline comments:
In `@packages/virtual-core/src/index.ts`:
- Line 667: Update the count-growing branch around nextCount > prevCount to
treat an update as an append only when the previous keys remain as an ordered
prefix; otherwise avoid scrollToEnd. Preserve isAppendWithTrim for
sliding-window updates, and add regression coverage for growing replacements and
reorders.

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📥 Commits

Reviewing files that changed from the base of the PR and between 789f5c2 and c452d48.

📒 Files selected for processing (5)

• .changeset/follow-sliding-window.md
• docs/api/virtualizer.md
• packages/virtual-core/src/index.ts
• packages/virtual-core/src/lazy-measurements.ts
• packages/virtual-core/tests/index.test.ts

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