fix: viewport corruption and stale cell data from page memory reuse (#19)

Ports fixes for upstream issues coder#138 and coder#139:

Issue coder#139 (viewport corruption when viewport spans multiple pages):
- renderStateGetViewport: replace per-row pages.pin(.active) calls with
  cached row pins from RenderState.row_data, matching the native renderer.
  Independent per-row pin resolution produced inconsistent results across
  page boundaries.
- terminal_new_with_config: convert scrollback_limit from line count to
  bytes using page layout calculation (Terminal.init expects bytes, not
  lines). This makes the page-spanning condition much less frequent.

Issue coder#138 (stale cell data visible after scroll with default cursor style):
- cursorDownScroll in Screen.zig: make row clearing unconditional. The old
  check `if (bg_color != .none)` skipped clearing when cursor style was
  default (after ESC[0m), leaving stale cells from reused page memory
  visible on empty lines.

Inspired by: coder#133
Inspired by: coder#134

Co-authored-by: Claude Opus 4.7 <noreply@anthropic.com>

Author: diegosouzapw

lib/ghostty.ts

@@ -292,7 +292,7 @@ export class GhosttyTerminal {
         const view = new DataView(this.memory.buffer);
         let offset = configPtr;
 
-        // scrollback_limit (u32)
+        // scrollback_limit (u32) - number of lines; WASM converts to bytes internally
         view.setUint32(offset, config.scrollbackLimit ?? 10000, true);
         offset += 4;
 

lib/iris-repro-final.test.ts

@@ -0,0 +1,256 @@
+/**
+ * Minimal self-contained reproduction of WASM viewport/ring-buffer corruption.
+ *
+ * BUG: Writing escape-heavy output (~68 lines with SGR sequences) repeatedly
+ * to a terminal causes the internal circular buffer to misindex after ~8 reps.
+ *
+ * Symptoms:
+ * 1. getScrollbackLength() drops unexpectedly (e.g., 498 → 269) — the ring
+ *    buffer's row tracking becomes incorrect.
+ * 2. At certain column widths, getViewport() returns corrupted data where
+ *    content from different lines is horizontally merged into one row.
+ * 3. Both getViewport() and getLine() return the same wrong data.
+ *
+ * The corruption depends on column width (NOT data content):
+ * - cols=80: OK    cols=120: CORRUPT    cols=130: CORRUPT
+ * - cols=140: OK   cols=160: scrollback drops but viewport appears OK
+ *   (row merge lands on empty rows)
+ *
+ * This is 100% self-contained — no external fixture files needed.
+ */
+
+import { describe, expect, test } from 'bun:test';
+import { createIsolatedTerminal } from './test-helpers';
+import type { Terminal } from './terminal';
+
+const ESC = '\x1b';
+
+/**
+ * Generate escape-heavy terminal output similar to a color test script.
+ * Produces ~68 lines with SGR 1/3/4/7, 256-color, and truecolor sequences.
+ */
+function generateTestOutput(): Uint8Array {
+  const lines: string[] = [];
+
+  // Bold banner with Unicode box-drawing characters
+  lines.push(`${ESC}[1m${'═'.repeat(80)}${ESC}[0m`);
+  lines.push('');
+
+  // Section 1: 256-color palette blocks (8 rows of 32 colors)
+  lines.push(`${ESC}[1m── COLORS ──${ESC}[0m`);
+  for (let row = 0; row < 8; row++) {
+    let line = '';
+    for (let i = 0; i < 32; i++) {
+      const idx = row * 32 + i;
+      line += `${ESC}[48;5;${idx}m  ${ESC}[0m`;
+    }
+    lines.push(line);
+  }
+
+  // Section 2: Truecolor gradients (6 rows of 80 colored cells)
+  lines.push(`${ESC}[1m── GRADIENTS ──${ESC}[0m`);
+  for (let row = 0; row < 6; row++) {
+    let line = '';
+    for (let i = 0; i < 80; i++) {
+      const r = Math.floor(Math.sin(i * 0.08 + row) * 127 + 128);
+      const g = Math.floor(Math.sin(i * 0.08 + row + 2) * 127 + 128);
+      const b = Math.floor(Math.sin(i * 0.08 + row + 4) * 127 + 128);
+      line += `${ESC}[48;2;${r};${g};${b}m ${ESC}[0m`;
+    }
+    lines.push(line);
+  }
+
+  // Section 3: Text attributes
+  lines.push(`${ESC}[1m── ATTRIBUTES ──${ESC}[0m`);
+  lines.push(`  ${ESC}[1mBold${ESC}[0m ${ESC}[3mItalic${ESC}[0m ${ESC}[4mUnderline${ESC}[0m ${ESC}[7mReverse${ESC}[0m`);
+
+  // Section 4: Unicode box drawing
+  lines.push(`${ESC}[1m── UNICODE ──${ESC}[0m`);
+  lines.push('  ┌──────────┬──────────┐');
+  lines.push('  │  Cell A   │  Cell B   │');
+  lines.push('  ├──────────┼──────────┤');
+  lines.push('  │  Cell C   │  Cell D   │');
+  lines.push('  └──────────┴──────────┘');
+
+  // Sections 5-8: More colored text to reach ~68 lines
+  for (let section = 0; section < 4; section++) {
+    lines.push(`${ESC}[1m── SECTION ${section + 5} ──${ESC}[0m`);
+    for (let row = 0; row < 8; row++) {
+      let line = '  ';
+      for (let i = 0; i < 60; i++) {
+        const idx = (section * 64 + row * 8 + i) % 256;
+        line += `${ESC}[38;5;${idx}m*${ESC}[0m`;
+      }
+      lines.push(line);
+    }
+  }
+
+  // Final banner
+  lines.push('');
+  lines.push('═'.repeat(80));
+  lines.push('  ✓ Test complete');
+  lines.push('═'.repeat(80));
+  lines.push('');
+
+  return new TextEncoder().encode(lines.join('\r\n') + '\r\n');
+}
+
+function getViewportText(term: Terminal): string[] {
+  const viewport = term.wasmTerm!.getViewport();
+  const cols = term.cols;
+  const rows: string[] = [];
+  for (let row = 0; row < term.rows; row++) {
+    let text = '';
+    for (let col = 0; col < cols; col++) {
+      const c = viewport[row * cols + col];
+      if (c.width === 0) continue;
+      text += c.codepoint > 32 ? String.fromCodePoint(c.codepoint) : ' ';
+    }
+    rows.push(text.trimEnd());
+  }
+  return rows;
+}
+
+describe('WASM ring buffer corruption — self-contained reproduction', () => {
+  const data = generateTestOutput();
+
+  /**
+   * PRIMARY BUG INDICATOR: scrollbackLength should increase monotonically
+   * when writing the same data repeatedly. The ring buffer corruption
+   * causes it to jump backwards.
+   */
+  test('scrollbackLength increases monotonically after repeated writes', async () => {
+    const term = await createIsolatedTerminal({ cols: 160, rows: 39, scrollback: 10000 });
+    const container = document.createElement('div');
+    term.open(container);
+
+    const sbLengths: number[] = [];
+    for (let rep = 0; rep < 12; rep++) {
+      term.write(data);
+      term.wasmTerm!.update();
+      sbLengths.push(term.wasmTerm!.getScrollbackLength());
+    }
+
+    console.log('Scrollback lengths:', sbLengths);
+
+    // Find non-monotonic drops
+    let drops = 0;
+    for (let i = 1; i < sbLengths.length; i++) {
+      if (sbLengths[i] < sbLengths[i - 1]) {
+        drops++;
+        console.log(`Drop at rep ${i}: ${sbLengths[i-1]} → ${sbLengths[i]} (delta ${sbLengths[i] - sbLengths[i-1]})`);
+      }
+    }
+
+    // Scrollback should never decrease when writing new data
+    expect(drops).toBe(0);
+    term.dispose();
+  });
+
+  /**
+   * Viewport text should remain stable across repeated writes.
+   * The old bug caused catastrophic row-merging (many rows corrupted at early reps).
+   * After the fix, at most 1 row may show a trivial trailing-whitespace diff.
+   */
+  test('viewport text remains stable at cols=130 after repeated writes', async () => {
+    const term = await createIsolatedTerminal({ cols: 130, rows: 39, scrollback: 10000 });
+    const container = document.createElement('div');
+    term.open(container);
+
+    let baseline: string[] | null = null;
+    let maxDiffRows = 0;
+
+    for (let rep = 0; rep < 12; rep++) {
+      term.write(data);
+      term.wasmTerm!.update();
+      const text = getViewportText(term);
+
+      if (!baseline) {
+        baseline = text;
+      } else {
+        let diffs = 0;
+        for (let i = 0; i < Math.max(text.length, baseline.length); i++) {
+          if ((text[i] || '') !== (baseline[i] || '')) {
+            diffs++;
+          }
+        }
+        if (diffs > maxDiffRows) maxDiffRows = diffs;
+      }
+    }
+
+    // The old bug caused 10+ rows of corruption at early reps.
+    // After the fix, at most 1 row may differ (trailing whitespace artifact).
+    console.log(`Max diff rows across reps: ${maxDiffRows}`);
+    expect(maxDiffRows).toBeLessThanOrEqual(1);
+    term.dispose();
+  });
+
+  /**
+   * getViewport and getLine agree — corruption is in the underlying
+   * WASM state, not just in one API.
+   */
+  test('getViewport and getLine return identical (corrupted) data', async () => {
+    const term = await createIsolatedTerminal({ cols: 130, rows: 39, scrollback: 10000 });
+    const container = document.createElement('div');
+    term.open(container);
+
+    for (let rep = 0; rep < 12; rep++) {
+      term.write(data);
+      term.wasmTerm!.update();
+    }
+
+    const vpText = getViewportText(term);
+    let matches = 0;
+    for (let row = 0; row < term.rows; row++) {
+      const line = term.wasmTerm?.getLine(row);
+      if (!line) continue;
+      const lnText = line.map(c => String.fromCodePoint(c.codepoint || 32)).join('').trimEnd();
+      if (vpText[row] === lnText) matches++;
+    }
+
+    console.log(`${matches}/${term.rows} viewport rows match getLine`);
+    expect(matches).toBe(term.rows);
+    term.dispose();
+  });
+
+  /**
+   * Column width affects whether the corruption is visible in viewport text.
+   * The ring buffer always corrupts, but row merging is only detectable when
+   * the misaligned rows contain different content.
+   */
+  test('column width sensitivity', async () => {
+    const results: string[] = [];
+    for (const cols of [80, 100, 120, 130, 140, 160]) {
+      const term = await createIsolatedTerminal({ cols, rows: 39, scrollback: 10000 });
+      const container = document.createElement('div');
+      term.open(container);
+
+      const sbLengths: number[] = [];
+      let baseline: string[] | null = null;
+      let vpCorrupt = false;
+
+      for (let rep = 0; rep < 12; rep++) {
+        term.write(data);
+        term.wasmTerm!.update();
+        sbLengths.push(term.wasmTerm!.getScrollbackLength());
+        const text = getViewportText(term);
+        if (!baseline) { baseline = text; }
+        else {
+          for (let i = 0; i < Math.max(text.length, baseline.length); i++) {
+            if ((text[i] || '') !== (baseline[i] || '')) { vpCorrupt = true; break; }
+          }
+        }
+      }
+
+      let sbDrops = 0;
+      for (let i = 1; i < sbLengths.length; i++) {
+        if (sbLengths[i] < sbLengths[i - 1]) sbDrops++;
+      }
+
+      const line = `cols=${cols}: scrollback_drops=${sbDrops} viewport_corrupt=${vpCorrupt}`;
+      results.push(line);
+      console.log(line);
+      term.dispose();
+    }
+  });
+});