Pre-existing diagnostics below an edit point used to surface as 'LSP
diagnostics introduced by this edit' whenever the edit deleted or
inserted lines. The delta-filter key included the diagnostic's
range, so the same logical error reported at a different line in
the post-edit snapshot looked like a brand new diagnostic.
Concrete case: deleting 14 lines in cli.py caused Pyright errors at
lines 9873, 10590, 12413, 13004 (unrelated to the edit) to be
reported as introduced by it.
Fix: build a piecewise-linear line-shift map (via difflib's
SequenceMatcher) from pre and post content, and remap baseline
diagnostics into post-edit coordinates before the set-difference.
Diagnostics in deleted regions drop out cleanly; diagnostics below
the edit shift by the right amount; diagnostics above are untouched.
The strict (range-aware) equality key stays — so a genuinely new
instance of an identical error class at a different line still
surfaces as new.
Pieces:
- agent/lsp/range_shift.py — build_line_shift, shift_diagnostic_range,
shift_baseline. Pure functions, no LSP state.
- agent/lsp/manager.py — LSPService.get_diagnostics_sync gains an
optional line_shift kwarg; baseline is shift_baseline'd before
computing the seen-set. _diag_key keeps the strict range key.
- tools/file_operations.py — write_file captures pre_content for any
LSP-handled extension (not just LINTERS_INPROC) and passes pre/post
to _maybe_lsp_diagnostics, which builds the shift map.
- New _lsp_handles_extension helper guards the pre_content read.
Trade-offs preserved:
- Genuinely new same-class errors at different lines still surface
(content-only key would have swallowed them).
- Pre-existing errors at unshifted positions still get filtered
(covered by the strict-key path with no shift).
- Best-effort: when pre_content can't be captured (file didn't
exist, permissions), the unshifted comparison still catches
most pre-existing errors; the edge case it misses is a new file
with a non-empty baseline, which is structurally impossible.
