Add synchronized output support (DEC mode 2026)

Implement synchronized output mode (CSI ? 2026 h/l) which allows
applications to batch terminal updates and render them atomically,
preventing screen tearing during rapid output.

Features:
- BSU (CSI ? 2026 h) pauses rendering, buffering row updates
- ESU (CSI ? 2026 l) flushes buffer and renders atomically
- Configurable timeout via synchronizedOutputTimeout option (default 5s)
- Exposed via terminal.modes.synchronizedOutputMode

Closes #3375

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <noreply@anthropic.com>

src/common/InputHandler.ts

@@ -1969,6 +1969,9 @@ export class InputHandler extends Disposable implements IInputHandler {
         case 2004: // bracketed paste mode (https://cirw.in/blog/bracketed-paste)
           this._coreService.decPrivateModes.bracketedPasteMode = true;
           break;
+        case 2026: // synchronized output (https://gist.github.com/christianparpart/d8a62cc1ab659194337d73e399004036)
+          this._coreService.decPrivateModes.synchronizedOutput = true;
+          break;
       }
     }
     return true;
@@ -2197,6 +2200,11 @@ export class InputHandler extends Disposable implements IInputHandler {
         case 2004: // bracketed paste mode (https://cirw.in/blog/bracketed-paste)
           this._coreService.decPrivateModes.bracketedPasteMode = false;
           break;
+        case 2026: // synchronized output (https://gist.github.com/christianparpart/d8a62cc1ab659194337d73e399004036)
+          this._coreService.decPrivateModes.synchronizedOutput = false;
+          // Trigger a full refresh now that the synchronized output block has ended
+          this._onRequestRefreshRows.fire(undefined);
+          break;
       }
     }
     return true;

src/common/TestUtils.test.ts

@@ -102,6 +102,7 @@ export class MockCoreService implements ICoreService {
     origin: false,
     reverseWraparound: false,
     sendFocus: false,
+    synchronizedOutput: false,
     wraparound: true
   };
   public onData: Event<string> = new Emitter<string>().event;

src/common/Types.ts

@@ -273,6 +273,7 @@ export interface IDecPrivateModes {
   origin: boolean;
   reverseWraparound: boolean;
   sendFocus: boolean;
+  synchronizedOutput: boolean;
   wraparound: boolean; // defaults: xterm - true, vt100 - false
 }
 

src/common/services/CoreService.ts

@@ -22,6 +22,7 @@ const DEFAULT_DEC_PRIVATE_MODES: IDecPrivateModes = Object.freeze({
   origin: false,
   reverseWraparound: false,
   sendFocus: false,
+  synchronizedOutput: false,
   wraparound: true // defaults: xterm - true, vt100 - false
 });
 

src/common/services/OptionsService.ts

@@ -37,6 +37,7 @@ export const DEFAULT_OPTIONS: Readonly<Required<ITerminalOptions>> = {
   scrollSensitivity: 1,
   screenReaderMode: false,
   smoothScrollDuration: 0,
+  synchronizedOutputTimeout: 5000,
   macOptionIsMeta: false,
   macOptionClickForcesSelection: false,
   minimumContrastRatio: 1,

src/common/services/Services.ts

@@ -259,6 +259,7 @@ export interface ITerminalOptions {
   scrollOnUserInput?: boolean;
   scrollSensitivity?: number;
   smoothScrollDuration?: number;
+  synchronizedOutputTimeout?: number;
   tabStopWidth?: number;
   theme?: ITheme;
   windowsMode?: boolean;