fix(lsp): changes file corruption

[ematipico]

Summary

Closes #9341

I used Claude Code for the fix.

Test Plan

Added a new test. Green CI

Docs

[changeset-bot]

🦋 Changeset detected

Latest commit: 29ce131

The changes in this PR will be included in the next version bump.

This PR includes changesets to release 13 packages

Name	Type
@biomejs/biome	Patch
@biomejs/cli-win32-x64	Patch
@biomejs/cli-win32-arm64	Patch
@biomejs/cli-darwin-x64	Patch
@biomejs/cli-darwin-arm64	Patch
@biomejs/cli-linux-x64	Patch
@biomejs/cli-linux-arm64	Patch
@biomejs/cli-linux-x64-musl	Patch
@biomejs/cli-linux-arm64-musl	Patch
@biomejs/wasm-web	Patch
@biomejs/wasm-bundler	Patch
@biomejs/wasm-nodejs	Patch
@biomejs/backend-jsonrpc	Patch

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📥 Commits

Reviewing files that changed from the base of the PR and between bc709f6 and 29ce131.

📒 Files selected for processing (2)

• .changeset/fix-lsp-char-boundary-crash.md
• crates/biome_lsp/src/utils.rs

---

Walkthrough

This pull request fixes an LSP crash that corrupted file content during save operations with format-on-save enabled. The root cause was improper handling of document change sequences where incremental edits preceded full document replacements. The fix modifies the apply_document_changes function to skip stale incremental changes when a subsequent full document replacement occurs, ensuring only relevant edits are applied. Regression tests validate the corrected behaviour with multi-byte UTF-8 content and offset handling.

Suggested labels

A-LSP, A-Formatter

🚥 Pre-merge checks | ✅ 4

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title directly refers to the core issue: fixing LSP-related file corruption during save operations.
Description check	✅ Passed	The description links to issue #9341, discloses AI assistance per guidelines, mentions test coverage, and confirms CI success.
Linked Issues check	✅ Passed	The PR fixes the LSP crash and file corruption by reworking apply_document_changes to skip stale incremental edits when a full document replacement occurs [#9341].
Out of Scope Changes check	✅ Passed	All changes—the changeset metadata, LSP utility fix, and regression tests—are directly scoped to resolving the file corruption bug reported in #9341.

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✨ Finishing Touches

• 📝 Generate docstrings (stacked PR)
• 📝 Generate docstrings (commit on current branch)

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Commit unit tests in branch fix/lsp-utils

📝 Coding Plan

• Generate coding plan for human review comments

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