fix(tui): fold rare prompt glyphs in the preview so they stop rendering as boxes

A beta tester photographed claude's `❯` prompt and its `⏵⏵` bypass-permissions
marker rendering as empty boxes in the preview pane. Their font has no coverage
for those codepoints while drawing `·`, `─`, `│` and `▶` perfectly.

The glyph TIER cannot help here. It answers "can this terminal do Unicode at
all", which is a locale question, and it correctly says yes for exactly the
terminals this affects. Coverage is per-glyph and undetectable from inside the
process, so the handful of rare glyphs CLIs use as chrome are folded to the
ASCII arrows they already look like, and everything a plain font does render is
left alone.

Scoped tightly: the preview only, never the TUI's own chrome, and skipped
entirely at the `nerd` tier where the user has declared a font that can draw
anything. The table is short and every entry was seen as tofu in a real
terminal rather than guessed at. The fold is length-preserving, so the preview
pane's column arithmetic is unaffected.

src/tui/tui-ansi.ts

@@ -331,6 +331,44 @@ function renderCells(cells: Cell[]): string {
  * Splitting matches `String.split('\n')`, so `''` yields `['']` and a trailing
  * newline yields a trailing empty line.
  */
+/**
+ * Glyphs a CLI draws as chrome that a plain terminal font very often has no
+ * coverage for, and the ASCII that means the same thing.
+ *
+ * ⚠️ This is NOT a substitute for the glyph TIER. The tier answers "can this
+ * terminal do Unicode at all", which is a locale question, and it says yes for
+ * exactly the terminals this table exists for: a beta tester's font rendered
+ * `·`, `─`, `│` and `▶` perfectly while drawing claude's `❯` prompt and its
+ * `⏵⏵` mode marker as empty boxes. Coverage is per-glyph and undetectable from
+ * here, so the rare ones are folded and the common ones are left alone.
+ *
+ * Kept deliberately SHORT. Every entry is a glyph seen rendering as tofu in a
+ * real terminal, not a guess, and each maps to the arrow it already looks like.
+ */
+const PREVIEW_GLYPH_FOLD: ReadonlyMap<string, string> = new Map([
+  ['\u276F', '>'], // ❯ heavy right-pointing angle quotation mark (claude, starship, zsh prompts)
+  ['\u276E', '<'], // ❮
+  ['\u23F5', '>'], // ⏵ black medium right-pointing triangle (claude's bypass-permissions marker)
+  ['\u23F4', '<'], // ⏴
+  ['\u23F6', '^'], // ⏶
+  ['\u23F7', 'v'], // ⏷
+  ['\u2771', '>'], // ❱
+  ['\u2770', '<'], // ❰
+]);
+
+/**
+ * Replace preview glyphs a plain font is likely to draw as an empty box.
+ *
+ * Applied to ANOTHER program's output on its way into the preview pane, never
+ * to the TUI's own chrome, and skipped at the `nerd` tier where the user has
+ * declared a font that can draw anything.
+ */
+export function foldPreviewGlyphs(line: string): string {
+  let out = '';
+  for (const char of line) out += PREVIEW_GLYPH_FOLD.get(char) ?? char;
+  return out;
+}
+
 export function toDisplayLines(raw: string): string[] {
   const lines: string[] = [];
   let cells: Cell[] = [];

src/tui/tui-app.ts

@@ -50,7 +50,7 @@ import chalk from 'chalk';
 import { palette, table, tint, type Tone } from '../cli-style.js';
 import { CODEMAN_INSTANCE, resolveTmuxSocketName } from '../config/instance.js';
 import { getErrorMessage } from '../types/api.js';
-import { dropSeveredEscape, toDisplayLines } from './tui-ansi.js';
+import { dropSeveredEscape, foldPreviewGlyphs, toDisplayLines } from './tui-ansi.js';
 import { approvalAnswerForKey, newApprovalIds } from './tui-approvals.js';
 import { composerScroll, composerStep, composerText, createComposer, type TuiComposerState } from './tui-composer.js';
 import { formatAwayDigest } from './tui-digest.js';
@@ -1315,7 +1315,12 @@ class TuiApp {
     try {
       const raw = await this.client.fetchTerminalTail(sessionId, PREVIEW_TAIL_BYTES);
       if (this.previewSessionId !== sessionId) return;
-      const lines = toDisplayLines(dropSeveredEscape(raw)).slice(-PREVIEW_MAX_LINES);
+      const tail = toDisplayLines(dropSeveredEscape(raw)).slice(-PREVIEW_MAX_LINES);
+      // A nerd font can draw anything; every other terminal gets the rare
+      // prompt glyphs folded to the arrows they already look like, because a
+      // font's per-glyph coverage cannot be detected from in here and tofu is
+      // worse than an ASCII arrow.
+      const lines = this.glyphTier === 'nerd' ? tail : tail.map(foldPreviewGlyphs);
       // An identical tail is what the backoff counts; anything new resets it, so
       // a pane that starts printing again is back to one read a second.
       if (this.applyPreview({ sessionId, lines })) this.previewQuiet = 0;

test/tui/tui-ansi.test.ts

@@ -16,6 +16,7 @@ import {
   toDisplayLines,
   visibleWidth,
   charWidth,
+  foldPreviewGlyphs,
 } from '../../src/tui/tui-ansi.js';
 
 const RED = '\x1b[31m';
@@ -204,3 +205,31 @@ describe('stripStyles', () => {
     expect(stripStyles('a\u{1f600}中')).toBe('a\u{1f600}中');
   });
 });
+
+describe('foldPreviewGlyphs', () => {
+  it("turns claude's prompt and mode markers into the arrows they look like", () => {
+    // Exactly what a beta tester photographed as empty boxes.
+    expect(foldPreviewGlyphs('\u276F Try "how does report_agent.py work?"')).toBe(
+      '> Try "how does report_agent.py work?"'
+    );
+    expect(foldPreviewGlyphs('  \u23F5\u23F5 bypass permissions on')).toBe('  >> bypass permissions on');
+  });
+
+  it('leaves the glyphs that plain fonts DO render', () => {
+    // The same terminal drew all of these correctly, so folding them would be a
+    // downgrade for everyone to fix a problem nobody has.
+    const kept = '\u00B7 \u2500 \u2502 \u25B6 \u25CB \u2714 \u2192';
+    expect(foldPreviewGlyphs(kept)).toBe(kept);
+  });
+
+  it('leaves ordinary text and box drawing exactly alone', () => {
+    const line = '  1 tui-demo-shell shell   \u25CB 4m\u2502';
+    expect(foldPreviewGlyphs(line)).toBe(line);
+    expect(foldPreviewGlyphs('')).toBe('');
+  });
+
+  it('preserves length, so preview column arithmetic is unaffected', () => {
+    const line = '\u276F hello \u23F5\u23F5 world';
+    expect(foldPreviewGlyphs(line)).toHaveLength([...line].length);
+  });
+});