ekp/postmortem/20260729-whole-hard-line-live-prefix.md

Let Live Editing Consume the Whole Hard-Line Plan

Superseded note (2026-07-29): the whole-hard-line planning decision remains current, but 20260729-editing-frontier-not-point.md supersedes this record's point-driven projection-boundary statements.

Later supersession (2026-07-29): 20260729-stable-live-transaction.md preserves whole-hard-line planning at structural commits but supersedes replanning after every source edit.

Context

The native progressive live model fixed the most visible editing problems: typing before native wrap stayed natural, narrow split windows soft-wrapped, and live mode no longer published discretionary hyphens or replacing breaks into the active tail.

User testing then exposed the next owner mistake. A hard line that naturally wraps into several visual rows still needs the rows before point to be decisions from one Knuth-Plass plan. The native-row model aligned rows after Emacs had already wrapped them, then treated those row boundaries as local facts. That cannot produce global KP behavior because later source text can change earlier optimal breaks and glue.

The failed assumption was subtle: "native redisplay owns the active edit tail" does not imply "native rows are the durable planning unit." Native redisplay should still own the point-containing unfinished area, but the completed prefix should be projected from the semantic KP plan for the whole current hard line.

Decision

Keep the core algorithm unchanged. ekp-layout-plan already computes the correct plan for complete text. ekp-buffer must consume that plan for the current hard line and choose the editable projection boundary.

The live pipeline is:

1. Read the complete source text of the current hard line.
2. Call the existing ekp-layout-plan with the authoritative width and current layout context.
3. Find the semantic plan line that contains point.
4. Project only complete plan lines before that line.
5. Leave the point-containing plan line and every later source character completely natural.
6. Compare old and new semantic line signatures so unchanged prefixes stay installed and changed suffixes are removed/reinstalled atomically.

This state belongs entirely to ekp-buffer: hard-line bounds, source and context signatures, the latest plan, line signatures, installed prefix range, generation, and a small buffer-local history cache for undo/redo and width/text round trips.

Boundaries

• Do not change core DP semantics.
• Do not add continuation DP, live DP, or final-line special cases.
• Do not change the C ABI, DP schema, or core/Elisp algorithm contract.
• Do not copy the DP into ekp-buffer.
• Do not add buffer, point, window, marker, or redisplay state to the core planner.
• Do not use overlays.
• Do not insert source spaces, source newlines, or source hyphen characters.

The completed-paragraph planner remains the owner of optimal breaks and glue. The buffer layer owns edit-time projection, invalidation, and redisplay safety.

Superseded Model

20260729-native-progressive-live-editing.md remains useful history for two decisions that still stand:

• the point-containing active area must remain natural;
• full static KP projection is allowed after hard-paragraph completion.

It is superseded for the planning unit. Completed native visual rows are not stable committed rows. They are only a symptom that there is now a semantic prefix before point that may be safe to project.

Consequences

• Earlier displayed rows in the same hard line can change together when later input changes the optimal KP plan.
• Moving point into an earlier semantic line immediately makes that line and the following source natural again.
• Point movement without text change still matters because it changes the projection boundary.
• Plan cache hits are possible when undo/redo, deletion, width changes, or point movement revisit a known hard-line text/context signature.
• Projection failure must not advance a tail marker or leave half of a KP prefix installed; the affected hard line returns to native display.
• First-line underfilled editing remains indistinguishable from ordinary Emacs because there is no complete semantic prefix before point.

Verification

The implementation proves the behavior through public editing paths, not by stubbing row lists:

• self-insert-command, yank, delete, real undo/redo, point movement, hard newline, resize, and major-mode change;
• point in last, middle, and first semantic plan line;
• early plan breaks changing after later edits, proving rows are not frozen;
• plan-cache hits and zero property writes when the semantic plan is unchanged;
• clean GUI dynamic evidence with exact source text, zero overlays, hscroll=0, natural point line, aligned prefix lines, and no delayed idle snap.

Implementation Outcome

task028 implemented this decision entirely in ekp-buffer. The core DP, C ABI, DP schema, and ekp-layout-plan contract were not changed. The live state now uses whole-hard-line plans, a 16-entry buffer-local history LRU, semantic line signatures, common-prefix differential publication, and transactional fail-closed cleanup.

Default and seed-20260729 ERT pass 181/181, all 181 tests pass in isolated Emacs processes, and C/Elisp fuzz passes 300/300. The reviewed main dynamic GUI run /private/tmp/ekp-semantic-live-v4-vFZTkr and 44-column split run /private/tmp/ekp-semantic-split-v3-uPwuOi both return PASS with exact source text, zero overlays, hscroll=0, a natural point line, and no temporal snap or stale projection.

The original C-backend run recorded append p99 6.399 ms with GC excluded. A later task029 audit could not reproduce that number with the current checked-in benchmark: the synthetic 80-pixel workload instead records one plan per unique append and roughly 33–85 ms p99. That separate performance debt is now issue018/task030; see 20260729-narrow-live-append-replanning.md. The implementation does not hide it with stale reuse, debounce, skipped publication, global GC changes, or a timer workaround.

Final independent code review returns APPROVE and independent architecture review returns CLEAR, including the theme-disable and frame-font invalidation delta.

Developer verification does not close issue016; the user must still confirm the editing experience personally.

Rollback

Revert the semantic-prefix live implementation as one unit if it fails its public-path or GUI gates. Do not restore native-row freezing as the final model; it is known to be the wrong planning boundary for global KP layout.