feat(viewer): join wrapped xterm rows for link matching

Adds joinWrappedRows/offsetToCell for reassembling a wrapped xterm
line into its full text plus offset->cell mapping, so link matching
can find paths that xterm has soft-wrapped across rows.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

app/src/lib/xtermLineJoin.test.ts

@@ -0,0 +1,49 @@
+import { describe, expect, it } from "vitest";
+import { joinWrappedRows, MAX_JOINED_LENGTH, offsetToCell, type RowSource } from "./xtermLineJoin";
+
+/** rows[i] = [text, isWrapped] */
+const buffer = (rows: Array<[string, boolean]>): RowSource => ({
+  getLine: (y) =>
+    rows[y] ? { isWrapped: rows[y][1], translateToString: (trim?: boolean) => (trim ? rows[y][0].trimEnd() : rows[y][0]) } : undefined,
+});
+
+describe("joinWrappedRows", () => {
+  it("returns a single unwrapped row as-is", () => {
+    const j = joinWrappedRows(buffer([["hello src/a.ts", false]]), 0);
+    expect(j).toEqual({ text: "hello src/a.ts", firstRow: 0, rowStarts: [0] });
+  });
+
+  it("walks up to the row that started the wrap and down through continuations", () => {
+    const b = buffer([
+      ["unrelated", false],
+      ["/workspace/very/long/pa", false],
+      ["th/to/file.ts:12 and mo", true],
+      ["re text", true],
+      ["next line", false],
+    ]);
+    const fromMiddle = joinWrappedRows(b, 2);
+    expect(fromMiddle.text).toBe("/workspace/very/long/path/to/file.ts:12 and more text");
+    expect(fromMiddle.firstRow).toBe(1);
+    expect(fromMiddle.rowStarts).toEqual([0, 23, 46]);
+    expect(joinWrappedRows(b, 1)).toEqual(fromMiddle);
+    expect(joinWrappedRows(b, 3)).toEqual(fromMiddle);
+  });
+
+  it("stops at the length budget", () => {
+    const rows: Array<[string, boolean]> = [["a".repeat(1000), false]];
+    for (let i = 0; i < 5; i++) rows.push(["b".repeat(1000), true]);
+    const j = joinWrappedRows(buffer(rows), 0);
+    expect(j.text.length).toBeLessThanOrEqual(MAX_JOINED_LENGTH);
+    expect(j.rowStarts.length).toBe(2);
+  });
+});
+
+describe("offsetToCell", () => {
+  it("maps offsets to 1-based cells on the right row", () => {
+    const j = { text: "abcdefgh", firstRow: 4, rowStarts: [0, 3, 6] };
+    expect(offsetToCell(j, 0)).toEqual({ x: 1, y: 5 });
+    expect(offsetToCell(j, 2)).toEqual({ x: 3, y: 5 });
+    expect(offsetToCell(j, 3)).toEqual({ x: 1, y: 6 });
+    expect(offsetToCell(j, 7)).toEqual({ x: 2, y: 7 });
+  });
+});

app/src/lib/xtermLineJoin.ts

@@ -0,0 +1,65 @@
+/**
+ * Joins an xterm buffer row with its wrapped continuations.
+ *
+ * `WebLinksAddon` does the same in its private `LinkComputer`, which the
+ * built package does not export — so the walk is repeated here, with the same
+ * 2048-character budget. Rows are read with `translateToString(true)`, which
+ * trims the right edge; a wrap never ends in trailing spaces xterm would keep,
+ * so the join is exact for the text a path can occur in.
+ *
+ * Wide characters (CJK, emoji) occupy two cells but one string index, so a
+ * column computed from a string offset drifts right of the glyph on such rows.
+ * The addon corrects this with `getCell`; v1 accepts the drift (underline
+ * lands a cell early; the click still resolves the same link).
+ */
+
+export const MAX_JOINED_LENGTH = 2048;
+
+/** Minimal slice of xterm's IBuffer this needs. */
+export interface RowSource {
+  getLine(y: number): { isWrapped: boolean; translateToString(trimRight?: boolean): string } | undefined;
+}
+
+export interface JoinedLine {
+  text: string;
+  /** 0-based index of the first buffer row that contributed. */
+  firstRow: number;
+  /** For each contributed row (in order), the string offset at which it starts. */
+  rowStarts: number[];
+}
+
+export function joinWrappedRows(buffer: RowSource, row: number): JoinedLine {
+  let top = row;
+  while (top > 0 && buffer.getLine(top)?.isWrapped) top--;
+
+  const parts: string[] = [];
+  let length = 0;
+  let y = top;
+  for (;;) {
+    const line = buffer.getLine(y);
+    if (!line) break;
+    if (y !== top && !line.isWrapped) break;
+    const text = line.translateToString(true);
+    if (length + text.length > MAX_JOINED_LENGTH && parts.length > 0) break;
+    parts.push(text);
+    length += text.length;
+    y++;
+  }
+
+  const rowStarts: number[] = [];
+  let offset = 0;
+  for (const p of parts) {
+    rowStarts.push(offset);
+    offset += p.length;
+  }
+  return { text: parts.join(""), firstRow: top, rowStarts };
+}
+
+/** String offset → 1-based {x, y} cell (y is the buffer row + 1). */
+export function offsetToCell(joined: JoinedLine, offset: number): { x: number; y: number } {
+  let rowIdx = 0;
+  for (let i = 0; i < joined.rowStarts.length; i++) {
+    if (joined.rowStarts[i] <= offset) rowIdx = i;
+  }
+  return { x: offset - joined.rowStarts[rowIdx] + 1, y: joined.firstRow + rowIdx + 1 };
+}