feat(render): pure decorationPlan feeds editor + webview from one diff (#64, INV-49)

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

src/trackChangesModel.ts

@@ -263,6 +263,42 @@ export function wordEditHunks(currentText: string, rewrittenText: string): EditH
   return hunks;
 }
 
+/** F12/#64 (INV-49/52): the editor render of one optimistically-applied proposal. */
+export interface DecorationPlan {
+  /** buffer ranges of inserted (proposed) text → tinted (INV-52). */
+  insertions: { start: number; end: number }[];
+  /** struck original text shown as a non-editable hint at a buffer offset (INV-52). */
+  deletions: { at: number; text: string }[];
+}
+
+/**
+ * F12/#64 (INV-49): compute the editor decoration plan for a proposal whose applied
+ * text occupies `[anchorStart, anchorStart+replacement.length)` in the buffer. The
+ * SAME word diff the webview uses (`wordEditHunks`) drives it, so both surfaces show
+ * the identical diff. A changed run maps to (a) an insertion range over the run's
+ * applied text and (b) a deletion hint carrying the run's removed original text at
+ * the run start; a pure insertion has no deletion hint; a pure deletion has only a
+ * hint. Pure, vscode-free, deterministic.
+ */
+export function decorationPlan(anchorStart: number, original: string, replacement: string): DecorationPlan {
+  const insertions: { start: number; end: number }[] = [];
+  const deletions: { at: number; text: string }[] = [];
+  // Walk the word diff once, tracking the applied-side offset as we consume parts.
+  let appliedOffset = anchorStart;
+  for (const part of diffWordsWithSpace(original, replacement)) {
+    if (part.added) {
+      insertions.push({ start: appliedOffset, end: appliedOffset + part.value.length });
+      appliedOffset += part.value.length;
+    } else if (part.removed) {
+      deletions.push({ at: appliedOffset, text: part.value });
+      // removed text is NOT in the applied buffer → appliedOffset does not advance
+    } else {
+      appliedOffset += part.value.length;
+    }
+  }
+  return { insertions, deletions };
+}
+
 const isWs = (c: string): boolean => /\s/.test(c);
 
 /**

test/decorationPlan.test.ts

@@ -0,0 +1,27 @@
+import { describe, test, expect } from "vitest";
+import { decorationPlan } from "../src/trackChangesModel";
+
+describe("decorationPlan (INV-49)", () => {
+  test("derives insertion ranges + deletion hints from original→replacement", () => {
+    // anchorStart 10; original 'the brown fox' → 'the red fox'
+    const plan = decorationPlan(10, "the brown fox", "the red fox");
+    // one changed run: 'brown ' → 'red ' (word-level); insertion 'red ' tinted,
+    // deletion hint 'brown ' shown at the run start.
+    expect(plan.insertions.length).toBeGreaterThan(0);
+    expect(plan.deletions.length).toBeGreaterThan(0);
+    // insertion offsets are in buffer coords (>= anchorStart) and lie within the applied text
+    for (const ins of plan.insertions) {
+      expect(ins.start).toBeGreaterThanOrEqual(10);
+      expect(ins.end).toBeLessThanOrEqual(10 + "the red fox".length);
+    }
+    // a deletion hint carries the struck original text at a buffer offset
+    expect(plan.deletions[0].text).toContain("brown");
+  });
+
+  test("pure insertion (no deletion) yields an insertion range and no deletion hint", () => {
+    // 'hello' → 'hello world': ' world' is a pure word-level insertion (no removed tokens)
+    const plan = decorationPlan(0, "hello", "hello world");
+    expect(plan.insertions.length).toBe(1);
+    expect(plan.deletions.length).toBe(0);
+  });
+});