fix(highlighter): preserve bounded syntax context

docs/issues/130-131-plan.md

@@ -0,0 +1,84 @@
+# Issues 130, 131, and 142 plan
+
+## Task
+
+Keep syntax-context work bounded across repeated distant viewport jumps, preserve injected Rust highlighting for fenced Markdown nested in block quotes, and stop conservative Rust overflow highlighting at truncated-line context barriers.
+
+## Current behavior
+
+The windowed Rust and Markdown highlighter keeps line-zero plus eight recent context checkpoints.
+
+Adding a checkpoint after a backward jump deletes every later checkpoint before enforcing the cap.
+
+Alternating between distant viewports can therefore rescan from line zero or another distant checkpoint on every jump.
+
+The Markdown checkpoint scanner recognizes fences only when the marker follows up to three leading spaces.
+
+A fence such as `> ```rust` is missed, and a deep viewport cannot synthesize the quoted fence opener needed for injected Rust highlighting.
+
+The #124 conservative fallback comment-classifies a complete rebuilt window when Rust block-comment nesting exceeds 64 levels.
+
+That fallback bypasses the truncated-line barriers used by context scanning and the normal parser path.
+
+Visible Rust code after an oversized line therefore remains incorrectly styled as a comment in plain Rust and fenced Markdown.
+
+## Implementation
+
+1. Keep line-zero plus a deterministic capped set of recently useful window-boundary checkpoints.
+2. Insert and refresh checkpoints without deleting valid checkpoints on the other side of a jump.
+3. Select the nearest valid checkpoint at or before a target independent of retention order.
+4. Invalidate only checkpoints after the first changed line while retaining the context at the start of that line.
+5. Recognize normalized block-quote container prefixes on Markdown fence openers.
+6. Retain the quote depth in Markdown fence context and apply it consistently to synthetic seeds, fenced contents, and closing fences.
+7. Leave a quoted fence when a later line no longer has its required container, then scan that line as ordinary Markdown.
+8. Split plain Rust overflow fallback at each oversized-line barrier and resume ordinary Rust parsing after the barrier.
+9. Stop fenced Markdown Rust overflow fallback at the first fence closer, oversized-line barrier, or loss of the required block-quote container.
+10. Preserve the fixed Rust block-comment overflow state and its 64-level exact wrapper cap from #124.
+
+## Acceptance criteria
+
+- After two distant viewports are warmed, deterministic alternation between them scans and parses work bounded independently of file depth.
+- Parser request bytes, retained highlight lines, and retained checkpoints keep deterministic caps.
+- A backward jump does not discard still-valid forward checkpoints.
+- An edit retains checkpoints at or before its first changed line and invalidates every later checkpoint whose syntax context may be stale.
+- After the affected side is rebuilt once, alternating across an edit remains bounded.
+- Shallow and deep block-quoted backtick and tilde fences retain Markdown fence styling and injected Rust highlighting.
+- Quoted fence openers, contents, and closers use the same block-quote container depth.
+- A quoted fence can close inside the visible window.
+- Editing a quote container invalidates and rebuilds the affected context correctly.
+- Plain Rust overflow fallback stops at the first truncated-line context barrier.
+- Ordinary Rust parsing and highlighting resume after that barrier within the same rebuilt window.
+- Fenced Markdown Rust overflow fallback stops at a truncated-line barrier without losing the surrounding quoted or unquoted fence context.
+- Quoted Markdown overflow fallback ends when the required block-quote container disappears, including when that line is oversized.
+- Edits before and around a truncated-line barrier invalidate and rebuild the affected window correctly.
+- Ordinary unquoted fences and Rust comments, strings, raw strings, and character literals remain correct.
+- The #124 nested Rust comment request bound and conservative overflow behavior remain unchanged in Rust and fenced Markdown.
+- Parser input, scanned work, retained checkpoints, and cached window size remain within the #112 and #124 bounds after overflow recovery.
+
+## Checks
+
+- Add focused regressions for alternating distant Rust and Markdown viewports, including an edit between the locations.
+- Assert scanned lines, request bytes, checkpoint count, and retained window size remain bounded after warm-up.
+- Add shallow and deep quoted-fence regressions for backticks and tildes with injected Rust.
+- Cover a quoted closer inside the visible window and edits that remove and restore a block-quote container.
+- Add plain Rust and fenced Markdown overflow regressions with 65 openers, an oversized barrier line, visible Rust code after it, and edits before and around the barrier.
+- Combine quoted overflow, a truncated-line barrier, and removal and restoration of the barrier's quote container.
+- Run focused highlighter tests.
+- Run every opt-in performance test with `cargo test performance:: -- --ignored --nocapture --test-threads=1`.
+- Run `cargo fmt --check`.
+- Run `git diff --check origin/main...HEAD`.
+- Run `cargo test --all-targets`.
+- Run `cargo clippy --all-targets --all-features -- -D warnings`.
+- Run `cargo build --all-targets`.
+- Apply the `improve` workflow to the final changed code without changing behavior.
+- Obtain fresh independent subagent review and address every important finding.
+- Wait for protected GitHub checks and current review feedback on the ready pull request.
+- Reply with commit and pull-request evidence on issues #130, #131, and #142.
+- Reply on the unresolved #141 review thread and resolve it only after the published fix fully addresses the report.
+
+## Out of scope
+
+- Background parsing.
+- A new parser framework or complete CommonMark parser.
+- LSP or semantic highlighting.
+- Changes to rendering, themes, editor commands, or the #124 overflow policy.