Add synchronized output support (DEC mode 2026)

demo/client.ts

@@ -1310,23 +1310,25 @@ function addVtButtons(): void {
   }
   const vtFragment = document.createDocumentFragment();
   const buttonSpecs: { [key: string]: { label: string, description: string, paramCount?: number }} = {
-    A:     { label: 'CUU ↑',    description: 'Cursor Up Ps Times' },
-    B:     { label: 'CUD ↓',    description: 'Cursor Down Ps Times' },
-    C:     { label: 'CUF →',    description: 'Cursor Forward Ps Times' },
-    D:     { label: 'CUB ←',    description: 'Cursor Backward Ps Times' },
-    E:     { label: 'CNL',      description: 'Cursor Next Line Ps Times' },
-    F:     { label: 'CPL',      description: 'Cursor Preceding Line Ps Times' },
-    G:     { label: 'CHA',      description: 'Cursor Character Absolute' },
-    H:     { label: 'CUP',      description: 'Cursor Position [row;column]', paramCount: 2 },
-    I:     { label: 'CHT',      description: 'Cursor Forward Tabulation Ps tab stops' },
-    J:     { label: 'ED',       description: 'Erase in Display' },
-    '?|J': { label: 'DECSED',   description: 'Erase in Display' },
-    K:     { label: 'EL',       description: 'Erase in Line' },
-    '?|K': { label: 'DECSEL',   description: 'Erase in Line' },
-    L:     { label: 'IL',       description: 'Insert Ps Line(s)' },
-    M:     { label: 'DL',       description: 'Delete Ps Line(s)' },
-    P:     { label: 'DCH',      description: 'Delete Ps Character(s)' },
-    ' q':  { label: 'DECSCUSR', description: 'Set Cursor Style', paramCount: 1 }
+    A:         { label: 'CUU ↑',    description: 'Cursor Up Ps Times' },
+    B:         { label: 'CUD ↓',    description: 'Cursor Down Ps Times' },
+    C:         { label: 'CUF →',    description: 'Cursor Forward Ps Times' },
+    D:         { label: 'CUB ←',    description: 'Cursor Backward Ps Times' },
+    E:         { label: 'CNL',      description: 'Cursor Next Line Ps Times' },
+    F:         { label: 'CPL',      description: 'Cursor Preceding Line Ps Times' },
+    G:         { label: 'CHA',      description: 'Cursor Character Absolute' },
+    H:         { label: 'CUP',      description: 'Cursor Position [row;column]', paramCount: 2 },
+    I:         { label: 'CHT',      description: 'Cursor Forward Tabulation Ps tab stops' },
+    J:         { label: 'ED',       description: 'Erase in Display' },
+    '?|J':     { label: 'DECSED',   description: 'Erase in Display' },
+    K:         { label: 'EL',       description: 'Erase in Line' },
+    '?|K':     { label: 'DECSEL',   description: 'Erase in Line' },
+    L:         { label: 'IL',       description: 'Insert Ps Line(s)' },
+    M:         { label: 'DL',       description: 'Delete Ps Line(s)' },
+    P:         { label: 'DCH',      description: 'Delete Ps Character(s)' },
+    ' q':      { label: 'DECSCUSR', description: 'Set Cursor Style' },
+    '?2026h':  { label: 'BSU',      description: 'Begin synchronized update', paramCount: 0 },
+    '?2026l':  { label: 'ESU',      description: 'End synchronized update', paramCount: 0 }
   };
   for (const s of Object.keys(buttonSpecs)) {
     const spec = buttonSpecs[s];

src/browser/public/Terminal.ts

@@ -123,6 +123,7 @@ export class Terminal extends Disposable implements ITerminalApi {
       originMode: m.origin,
       reverseWraparoundMode: m.reverseWraparound,
       sendFocusMode: m.sendFocus,
+      synchronizedOutputMode: m.synchronizedOutput,
       wraparoundMode: m.wraparound
     };
   }

src/browser/services/RenderService.ts

@@ -9,7 +9,7 @@ import { IRenderDimensions, IRenderer } from 'browser/renderer/shared/Types';
 import { ICharSizeService, ICoreBrowserService, IRenderService, IThemeService } from 'browser/services/Services';
 import { Disposable, MutableDisposable, toDisposable } from 'vs/base/common/lifecycle';
 import { DebouncedIdleTask } from 'common/TaskQueue';
-import { IBufferService, IDecorationService, IOptionsService } from 'common/services/Services';
+import { IBufferService, ICoreService, IDecorationService, IOptionsService } from 'common/services/Services';
 import { Emitter } from 'vs/base/common/event';
 
 interface ISelectionState {
@@ -18,6 +18,10 @@ interface ISelectionState {
   columnSelectMode: boolean;
 }
 
+const enum Constants {
+  SYNCHRONIZED_OUTPUT_TIMEOUT_MS = 1000
+}
+
 export class RenderService extends Disposable implements IRenderService {
   public serviceBrand: undefined;
 
@@ -32,6 +36,7 @@ export class RenderService extends Disposable implements IRenderService {
   private _needsSelectionRefresh: boolean = false;
   private _canvasWidth: number = 0;
   private _canvasHeight: number = 0;
+  private _syncOutputHandler: SynchronizedOutputHandler;
   private _selectionState: ISelectionState = {
     start: undefined,
     end: undefined,
@@ -52,23 +57,31 @@ export class RenderService extends Disposable implements IRenderService {
   constructor(
     private _rowCount: number,
     screenElement: HTMLElement,
-    @IOptionsService optionsService: IOptionsService,
+    @IOptionsService private readonly _optionsService: IOptionsService,
     @ICharSizeService private readonly _charSizeService: ICharSizeService,
+    @ICoreService private readonly _coreService: ICoreService,
     @IDecorationService decorationService: IDecorationService,
     @IBufferService bufferService: IBufferService,
-    @ICoreBrowserService coreBrowserService: ICoreBrowserService,
+    @ICoreBrowserService private readonly _coreBrowserService: ICoreBrowserService,
     @IThemeService themeService: IThemeService
   ) {
     super();
 
-    this._renderDebouncer = new RenderDebouncer((start, end) => this._renderRows(start, end), coreBrowserService);
+    this._renderDebouncer = new RenderDebouncer((start, end) => this._renderRows(start, end), this._coreBrowserService);
     this._register(this._renderDebouncer);
 
-    this._register(coreBrowserService.onDprChange(() => this.handleDevicePixelRatioChange()));
+    this._syncOutputHandler = new SynchronizedOutputHandler(
+      this._coreBrowserService,
+      this._coreService,
+      () => this._fullRefresh()
+    );
+    this._register(toDisposable(() => this._syncOutputHandler.dispose()));
+
+    this._register(this._coreBrowserService.onDprChange(() => this.handleDevicePixelRatioChange()));
 
     this._register(bufferService.onResize(() => this._fullRefresh()));
     this._register(bufferService.buffers.onBufferActivate(() => this._renderer.value?.clear()));
-    this._register(optionsService.onOptionChange(() => this._handleOptionsChanged()));
+    this._register(this._optionsService.onOptionChange(() => this._handleOptionsChanged()));
     this._register(this._charSizeService.onCharSizeChange(() => this.handleCharSizeChanged()));
 
     // Do a full refresh whenever any decoration is added or removed. This may not actually result
@@ -78,7 +91,7 @@ export class RenderService extends Disposable implements IRenderService {
     this._register(decorationService.onDecorationRemoved(() => this._fullRefresh()));
 
     // Clear the renderer when the a change that could affect glyphs occurs
-    this._register(optionsService.onMultipleOptionChange([
+    this._register(this._optionsService.onMultipleOptionChange([
       'customGlyphs',
       'drawBoldTextInBrightColors',
       'letterSpacing',
@@ -96,15 +109,15 @@ export class RenderService extends Disposable implements IRenderService {
     }));
 
     // Refresh the cursor line when the cursor changes
-    this._register(optionsService.onMultipleOptionChange([
+    this._register(this._optionsService.onMultipleOptionChange([
       'cursorBlink',
       'cursorStyle'
     ], () => this.refreshRows(bufferService.buffer.y, bufferService.buffer.y, true)));
 
     this._register(themeService.onChangeColors(() => this._fullRefresh()));
 
-    this._registerIntersectionObserver(coreBrowserService.window, screenElement);
-    this._register(coreBrowserService.onWindowChange((w) => this._registerIntersectionObserver(w, screenElement)));
+    this._registerIntersectionObserver(this._coreBrowserService.window, screenElement);
+    this._register(this._coreBrowserService.onWindowChange((w) => this._registerIntersectionObserver(w, screenElement)));
   }
 
   private _registerIntersectionObserver(w: Window & typeof globalThis, screenElement: HTMLElement): void {
@@ -137,6 +150,18 @@ export class RenderService extends Disposable implements IRenderService {
       this._needsFullRefresh = true;
       return;
     }
+
+    if (this._coreService.decPrivateModes.synchronizedOutput) {
+      this._syncOutputHandler.bufferRows(start, end);
+      return;
+    }
+
+    const buffered = this._syncOutputHandler.flush();
+    if (buffered) {
+      start = Math.min(start, buffered.start);
+      end = Math.max(end, buffered.end);
+    }
+
     if (!isRedrawOnly) {
       this._isNextRenderRedrawOnly = false;
     }
@@ -148,6 +173,13 @@ export class RenderService extends Disposable implements IRenderService {
       return;
     }
 
+    // Skip rendering if synchronized output mode is enabled. This check must happen here
+    // (in addition to refreshRows) to handle renders that were queued before the mode was enabled.
+    if (this._coreService.decPrivateModes.synchronizedOutput) {
+      this._syncOutputHandler.bufferRows(start, end);
+      return;
+    }
+
     // Since this is debounced, a resize event could have happened between the time a refresh was
     // requested and when this triggers. Clamp the values of start and end to ensure they're valid
     // given the current viewport state.
@@ -283,3 +315,62 @@ export class RenderService extends Disposable implements IRenderService {
     this._renderer.value?.clear();
   }
 }
+
+/**
+ * Buffers row refresh requests during synchronized output mode (DEC mode 2026).
+ * When the mode is disabled, the accumulated row range is flushed for rendering.
+ * A safety timeout ensures rendering occurs even if the end sequence is not received.
+ */
+class SynchronizedOutputHandler {
+  private _start: number = 0;
+  private _end: number = 0;
+  private _timeout: number | undefined;
+  private _isBuffering: boolean = false;
+
+  constructor(
+    private readonly _coreBrowserService: ICoreBrowserService,
+    private readonly _coreService: ICoreService,
+    private readonly _onTimeout: () => void
+  ) {}
+
+  public bufferRows(start: number, end: number): void {
+    if (!this._isBuffering) {
+      this._start = start;
+      this._end = end;
+      this._isBuffering = true;
+    } else {
+      this._start = Math.min(this._start, start);
+      this._end = Math.max(this._end, end);
+    }
+
+    if (this._timeout === undefined) {
+      this._timeout = this._coreBrowserService.window.setTimeout(() => {
+        this._timeout = undefined;
+        this._coreService.decPrivateModes.synchronizedOutput = false;
+        this._onTimeout();
+      }, Constants.SYNCHRONIZED_OUTPUT_TIMEOUT_MS);
+    }
+  }
+
+  public flush(): { start: number, end: number } | undefined {
+    if (this._timeout !== undefined) {
+      this._coreBrowserService.window.clearTimeout(this._timeout);
+      this._timeout = undefined;
+    }
+
+    if (!this._isBuffering) {
+      return undefined;
+    }
+
+    const result = { start: this._start, end: this._end };
+    this._isBuffering = false;
+    return result;
+  }
+
+  public dispose(): void {
+    if (this._timeout !== undefined) {
+      this._coreBrowserService.window.clearTimeout(this._timeout);
+      this._timeout = undefined;
+    }
+  }
+}

src/common/InputHandler.test.ts

@@ -2314,7 +2314,7 @@ describe('InputHandler', () => {
     });
     it('DEC privates with set/reset semantic', async () => {
       // initially reset
-      const reset = [1, 6, 9, 12, 45, 66, 1000, 1002, 1003, 1004, 1006, 1016, 47, 1047, 1049, 2004];
+      const reset = [1, 6, 9, 12, 45, 66, 1000, 1002, 1003, 1004, 1006, 1016, 47, 1047, 1049, 2004, 2026];
       for (const mode of reset) {
         await inputHandler.parseP(`\x1b[?${mode}$p`);
         assert.deepEqual(reportStack.pop(), `\x1b[?${mode};2$y`);   // initial reset

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://github.com/contour-terminal/vt-extensions/blob/master/synchronized-output.md)
+          this._coreService.decPrivateModes.synchronizedOutput = true;
+          break;
       }
     }
     return true;
@@ -2197,6 +2200,10 @@ 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://github.com/contour-terminal/vt-extensions/blob/master/synchronized-output.md)
+          this._coreService.decPrivateModes.synchronizedOutput = false;
+          this._onRequestRefreshRows.fire(undefined);
+          break;
       }
     }
     return true;
@@ -2291,6 +2298,7 @@ export class InputHandler extends Disposable implements IInputHandler {
     if (p === 1048) return f(p, V.SET); // xterm always returns SET here
     if (p === 47 || p === 1047 || p === 1049) return f(p, b2v(active === alt));
     if (p === 2004) return f(p, b2v(dm.bracketedPasteMode));
+    if (p === 2026) return f(p, b2v(dm.synchronizedOutput));
     return f(p, V.NOT_RECOGNIZED);
   }
 

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
 });
 

test/playwright/SharedRendererTests.ts

@@ -1267,6 +1267,45 @@ export function injectSharedRendererTests(ctx: ISharedRendererTestContext): void
       await pollFor(ctx.value.page, () => getCellColor(ctx.value, 1, 1), [128, 0, 0, 255]);
     });
   });
+
+  test.describe('synchronized output', () => {
+    test('defers rendering until ESU', async () => {
+      await ctx.value.proxy.write('\x1b[?2026h'); // BSU
+      await ctx.value.proxy.write('\x1b[31m■');
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 1, 1), [0, 0, 0, 255]);
+      await ctx.value.proxy.write('\x1b[?2026l'); // ESU
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 1, 1), [205, 49, 49, 255]);
+    });
+
+    test('batches multiple writes', async () => {
+      await ctx.value.proxy.write('\x1b[?2026h'); // BSU
+      await ctx.value.proxy.write('\x1b[31m■\x1b[32m■\x1b[34m■');
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 1, 1), [0, 0, 0, 255]);
+      await ctx.value.proxy.write('\x1b[?2026l'); // ESU
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 1, 1), [205, 49, 49, 255]);
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 2, 1), [13, 188, 121, 255]);
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 3, 1), [36, 114, 200, 255]);
+    });
+
+    test('nested BSU is idempotent', async () => {
+      await ctx.value.proxy.write('\x1b[?2026h'); // BSU
+      await ctx.value.proxy.write('\x1b[31m■');
+      await ctx.value.proxy.write('\x1b[?2026h'); // BSU
+      await ctx.value.proxy.write('\x1b[32m■');
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 1, 1), [0, 0, 0, 255]);
+      await ctx.value.proxy.write('\x1b[?2026l'); // ESU
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 1, 1), [205, 49, 49, 255]);
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 2, 1), [13, 188, 121, 255]);
+    });
+
+    test('timeout flushes without ESU', async () => {
+      await ctx.value.proxy.write('\x1b[?2026h'); // BSU
+      await ctx.value.proxy.write('\x1b[31m■');
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 1, 1), [0, 0, 0, 255]);
+      await ctx.value.page.waitForTimeout(1500);
+      await pollFor(ctx.value.page, () => getCellColor(ctx.value, 1, 1), [205, 49, 49, 255]);
+    });
+  });
 }
 
 enum CellColorPosition {

test/playwright/Terminal.test.ts

@@ -609,6 +609,7 @@ test.describe('API Integration Tests', () => {
         originMode: false,
         reverseWraparoundMode: false,
         sendFocusMode: false,
+        synchronizedOutputMode: false,
         wraparoundMode: true
       });
     });

typings/xterm.d.ts

@@ -1968,6 +1968,13 @@ declare module '@xterm/xterm' {
      * Send FocusIn/FocusOut events: `CSI ? 1 0 0 4 h`
      */
     readonly sendFocusMode: boolean;
+    /**
+     * Synchronized Output Mode: `CSI ? 2 0 2 6 h`
+     *
+     * When enabled, output is buffered and only rendered when the mode is
+     * disabled, allowing for atomic screen updates without tearing.
+     */
+    readonly synchronizedOutputMode: boolean;
     /**
      * Auto-Wrap Mode (DECAWM): `CSI ? 7 h`
      */