ekp/postmortem/20260729-editing-frontier-not-point.md

Let Source Edits Own the Live Editing Frontier

Superseded note (2026-07-29): point-only motion remains a zero-work invariant. 20260729-stable-live-transaction.md replaces the frontier's natural-suffix and immediate-publication ownership with a saved committed projection and a local dirty edit transaction.

Context

task028 fixed the planning boundary: all projected rows in one hard line now come from one complete ekp-layout-plan. It nevertheless used current point to choose the natural suffix whenever post-command-hook ran.

That conflated two independent facts:

• point is transient navigation state;
• the editing frontier records which semantic line is currently unfinished.

As a result, cursor motion alone removed and reinstalled projection properties. It did no DP work, but it still changed the visual document. Worse, a later width or font reflow could read the cursor's temporary position and silently move the natural suffix boundary.

Decision

The latest real source edit owns the editing frontier.

1. A live hard line keeps a source marker for the changed region's new end.
2. Before a real edit, the existing prefix line containing the edit is returned to native display together with its suffix.
3. After the edit, the frontier marker moves to the changed region's new end and the whole-hard-line plan is recomputed or reused once.
4. The active semantic index is derived from that marker, never from transient point.
5. Point-only motion within the active hard line does nothing.
6. Width, font, and layout-context changes preserve the marker and map it into the new plan.
7. Leaving the hard line still completes the old hard line with the existing static KP path.

The core DP remains unaware of point, markers, buffers, and editing state.

Why This Is Simpler

active-index is derived plan state; the frontier marker is the one stable source fact. There is no point-motion state machine and no attempt to infer editing intent from navigation. One owner event—source mutation—moves the frontier.

The change deletes the point-boundary publication path instead of adding another condition to it.

Superseded Decision

This record supersedes only the point-driven boundary statements in 20260729-whole-hard-line-live-prefix.md. That record's main decisions remain valid:

• one complete hard line is the planning unit;
• all projected prefix lines come from the same core plan;
• the frontier line and suffix stay natural;
• prior breaks may change together after later source edits.

Consequences

• Moving point cannot cause visual reflow or text-property writes.
• Editing an earlier projected line still makes that actual edit line and its suffix natural before mutation, then republishes from the updated whole-hard-line plan.
• Resizing can change breaks and glue, but not because point happened to be elsewhere inside the hard line.
• The frontier remains display-layer state. Core DP, the C ABI, and the source-clean representation are unchanged.

Verification

The regression gate compares the complete owned-property projection, frontier marker, active index, signatures, spans, source characters, modified tick, undo state, plan calls, cache calls, and property writes before and after backward/forward point motion. It failed before the change and now passes; reflow and deferred IME completion also preserve the frontier instead of reading transient point.

Default, seed-20260729, and isolated full ERT pass 182/182; buffer ERT passes 93/93 and C/Elisp fuzz passes 300/300. Point-motion p99 is 0.033 ms on the C backend with zero plan/cache calls.

Reviewed dynamic GUI evidence at /private/tmp/ekp-frontier-live-v3-66WYRW contains a 26.6-second screen recording and 39 manifest lines. Backward/forward commands preserve the projection hash, plan, generation, cache size, active index, and frontier through every required checkpoint. Delete, yank, real undo, resize, restore, and hard-newline transitions also pass; source text remains exact, overlays and horizontal scroll remain zero, and the final report is PASS.

Independent architecture review is CLEAR. Independent code review finds zero task029 blockers; the separate unique-append performance debt is recorded as issue018/task030.

Rollback

Revert this editing-frontier change as one unit if a public editing or GUI gate fails. Do not restore point-driven projection changes as a fallback; navigation is not a layout invalidation event.