fix: cap unlimited terminal scrollback

components/terminal/runtime/createXTermRuntime.ts

@@ -15,6 +15,7 @@ import { fontStore } from "../../../application/state/fontStore";
 import { KeywordHighlighter } from "../keywordHighlight";
 import {
   XTERM_PERFORMANCE_CONFIG,
+  resolveXTermScrollback,
   type XTermPlatform,
   resolveXTermPerformanceConfig,
 } from "../../../infrastructure/config/xtermPerformance";
@@ -270,11 +271,8 @@ export const createXTermRuntime = (ctx: CreateXTermRuntimeContext): XTermRuntime
 
   const cursorStyle = settings?.cursorShape ?? "block";
   const cursorBlink = settings?.cursorBlink ?? true;
-  // xterm.js treats scrollback=0 as "no scrollback buffer", which breaks mouse
-  // wheel scrolling (events become arrow-key sequences).  The UI uses 0 to mean
-  // "no limit", so map it to a large value instead.
   const rawScrollback = settings?.scrollback ?? 10000;
-  const scrollback = rawScrollback === 0 ? 999999 : rawScrollback;
+  const scrollback = resolveXTermScrollback(rawScrollback);
   const drawBoldTextInBrightColors = settings?.drawBoldInBrightColors ?? true;
   const fontWeight = resolveHostTerminalFontWeight(ctx.host, settings?.fontWeight ?? 400);
   const fontWeightBold = settings?.fontWeightBold ?? 700;

components/terminal/useTerminalEffects.ts

@@ -1,6 +1,7 @@
 /* eslint-disable @typescript-eslint/no-explicit-any, react-hooks/exhaustive-deps */
 import { useRef } from 'react';
 import { resolveFontWeightBold } from '../../lib/fontWeightAvailability';
+import { resolveXTermScrollback } from '../../infrastructure/config/xtermPerformance';
 import { shouldInterceptMouseTrackingContextMenu } from './runtime/middleClickBehavior';
 
 type TerminalEffectsContext = Record<string, any>;
@@ -465,7 +466,7 @@ export function useTerminalEffects(ctx: TerminalEffectsContext) {
 
     if (terminalSettings) {
       applyUserCursorPreference(termRef.current, terminalSettings);
-      termRef.current.options.scrollback = terminalSettings.scrollback === 0 ? 999999 : terminalSettings.scrollback;
+      termRef.current.options.scrollback = resolveXTermScrollback(terminalSettings.scrollback);
       termRef.current.options.fontWeight = effectiveFontWeight as
         | 100
         | 200

infrastructure/config/xtermPerformance.test.ts

@@ -0,0 +1,17 @@
+import test from 'node:test';
+import assert from 'node:assert/strict';
+
+import {
+  XTERM_UNLIMITED_SCROLLBACK_CAP,
+  resolveXTermScrollback,
+} from './xtermPerformance';
+
+test('resolveXTermScrollback maps the unlimited sentinel to a 50000 row cap', () => {
+  assert.equal(XTERM_UNLIMITED_SCROLLBACK_CAP, 50000);
+  assert.equal(resolveXTermScrollback(0), 50000);
+});
+
+test('resolveXTermScrollback preserves explicit positive scrollback values', () => {
+  assert.equal(resolveXTermScrollback(10000), 10000);
+  assert.equal(resolveXTermScrollback(50000), 50000);
+});

infrastructure/config/xtermPerformance.ts

@@ -8,6 +8,14 @@
  * - Memory pressure handling
  */
 
+export const XTERM_UNLIMITED_SCROLLBACK_CAP = 50000;
+
+export function resolveXTermScrollback(scrollback: number): number {
+  // xterm.js treats 0 as "no scrollback". Keep the app's 0 sentinel useful
+  // without asking xterm to resize/reflow nearly one million buffer rows.
+  return scrollback === 0 ? XTERM_UNLIMITED_SCROLLBACK_CAP : scrollback;
+}
+
 export const XTERM_PERFORMANCE_CONFIG = {
   // Memory and Scrollback Settings
   scrollback: {