## Bug
**Sentry:**
[ZED-3W3](https://sentry.io/organizations/zed-dev/issues/7085300207/)
(13 events, first seen 2025-12-03)
`summaries_for_anchors` panics with "cannot seek backward" when
iterating selection anchors whose excerpt locators are no longer in
ascending order after an `update_path_excerpts` call.
### Root cause
`update_path_excerpts` merges new excerpt ranges with existing ones.
When an existing excerpt has no overlap with any new range (e.g.
`existing_range.end < new.context.start`), it is removed from the
excerpts tree but is **not** recorded in `replaced_excerpts`. Its stale
locator persists in the `excerpt_ids` tree.
Later, when a new excerpt is inserted for a different path whose locator
falls between the stale locator and another surviving excerpt,
`summaries_for_anchors` can encounter anchors in an order where it needs
to seek the cursor *backward* — violating the forward-only cursor
invariant.
Concretely, the scenario requires:
1. A path with ≥3 excerpts (E_B1, E_B2, E_B3) with anchors in E_B2 and
E_B3.
2. An `update_path_excerpts` call that keeps E_B1, **removes E_B2** (no
overlap → no `replaced_excerpts` entry), and replaces E_B3 with a new
excerpt N.
3. A new path D inserted between paths B and C, whose excerpt E_D gets a
locator between N and E_B2's stale locator.
4. Resolving the old anchors: E_B2's stale locator seeks past E_D, then
E_B3→N tries to seek backward to N — **panic**.
## Fix
In the two branches of `update_path_excerpts` where existing excerpts
are removed without being absorbed into a new range, map the removed
excerpt to the nearest surviving neighbor (the last kept or inserted
excerpt) in `replaced_excerpts`. This ensures stale anchors always remap
to a valid, correctly-ordered locator.
The two branches are:
- `existing_range.end < new.context.start` — existing excerpt falls
entirely before the next new range
- `(None, Some(...))` — no more new ranges for remaining existing
excerpts
## Verification
- A reproduction test
(`test_cannot_seek_backward_after_excerpt_replacement`) that constructs
the exact crash scenario now passes.
- Full `multi_buffer` test suite (48 tests) passes with no regressions.
- Clippy is clean.
Release Notes:
- Fixed a possible crash when updating excerpts in multibuffers
Eric Holk
53958dad4e