feat(diff): represent expanded context in row geometry

Model folded context as explicit diff rows before render planning so line gutters, hunk anchors, row windowing, and section geometry all share the same structure.

Author: adit-chandra

src/ui/diff/codeColumns.test.ts

@@ -1,6 +1,7 @@
 import { describe, expect, test } from "bun:test";
 import type { DiffFile } from "../../core/types";
-import { maxFileCodeLineWidth } from "./codeColumns";
+import { findMaxLineNumberInRows, maxFileCodeLineWidth } from "./codeColumns";
+import type { DiffRow } from "./pierre";
 
 /** Generate a large diff metadata fixture without checking a huge file into the repo. */
 function createLargeLineFixture(lineCount: number, widestLine: string): DiffFile {
@@ -29,3 +30,51 @@ describe("code column measurement", () => {
     expect(maxFileCodeLineWidth(file)).toBe("the widest generated line".length);
   });
 });
+
+describe("findMaxLineNumberInRows", () => {
+  test("accounts for collapsed gap ranges that can later expand", () => {
+    const rows: DiffRow[] = [
+      {
+        type: "split-line",
+        key: "file:line:0",
+        fileId: "file",
+        hunkIndex: 0,
+        left: { kind: "deletion", sign: "-", lineNumber: 5, spans: [{ text: "old" }] },
+        right: { kind: "addition", sign: "+", lineNumber: 5, spans: [{ text: "new" }] },
+      },
+      {
+        type: "collapsed",
+        key: "file:collapsed:trailing",
+        fileId: "file",
+        hunkIndex: 0,
+        oldRange: [6, 1000],
+        newRange: [6, 1000],
+        position: "trailing",
+        text: "995 unchanged lines",
+      },
+    ];
+
+    expect(findMaxLineNumberInRows(rows)).toBe(1000);
+  });
+
+  test("accounts for synthesized stack expansion rows", () => {
+    const rows: DiffRow[] = [
+      {
+        type: "stack-line",
+        key: "file:expanded:trailing:0:0",
+        fileId: "file",
+        hunkIndex: 0,
+        isExpansionRow: true,
+        cell: {
+          kind: "context",
+          sign: " ",
+          oldLineNumber: 998,
+          newLineNumber: 1002,
+          spans: [{ text: "context" }],
+        },
+      },
+    ];
+
+    expect(findMaxLineNumberInRows(rows, 9)).toBe(1002);
+  });
+});

src/ui/diff/codeColumns.ts

@@ -1,4 +1,5 @@
 import type { DiffFile, LayoutMode } from "../../core/types";
+import type { DiffRow } from "./pierre";
 
 export const DIFF_CODE_TAB_WIDTH = 2;
 export const DIFF_RAIL_PREFIX_WIDTH = 1;
@@ -47,6 +48,29 @@ export function findMaxLineNumber(file: DiffFile) {
   return Math.max(highest, 1);
 }
 
+/** Find the widest line-number gutter needed for an already-expanded row stream. */
+export function findMaxLineNumberInRows(rows: Iterable<DiffRow>, fallback = 1) {
+  let highest = fallback;
+
+  for (const row of rows) {
+    if (row.type === "collapsed") {
+      highest = Math.max(highest, row.oldRange[1], row.newRange[1]);
+      continue;
+    }
+
+    if (row.type === "split-line") {
+      highest = Math.max(highest, row.left.lineNumber ?? 0, row.right.lineNumber ?? 0);
+      continue;
+    }
+
+    if (row.type === "stack-line") {
+      highest = Math.max(highest, row.cell.oldLineNumber ?? 0, row.cell.newLineNumber ?? 0);
+    }
+  }
+
+  return Math.max(highest, 1);
+}
+
 /** Split-view panes reserve one rail column on the left and one separator column in the middle. */
 export function resolveSplitPaneWidths(width: number) {
   const usableWidth = Math.max(0, width - DIFF_RAIL_PREFIX_WIDTH - DIFF_SPLIT_SEPARATOR_WIDTH);