Add synchronized output support (DEC mode 2026) by chrislloyd · Pull Request #5453 · xtermjs/xterm.js

chrislloyd · 2025-12-04T19:17:31Z

Summary

Implements synchronized output mode (DEC private mode 2026) as specified in the VT extension.

BSU (CSI ? 2026 h) - Begin Synchronized Update: defers rendering
ESU (CSI ? 2026 l) - End Synchronized Update: flushes buffer atomically
DECRQM (CSI ? 2026 $ p) - Query mode support

This allows applications to batch terminal updates and render them atomically, preventing screen tearing during rapid output.

Features

Rendering is deferred when mode 2026 is enabled
Row updates are tracked and rendered atomically when mode is disabled
Safety timeout of 1 second auto-flushes if ESU is not received
Feature detection via DECRQM query

Test Plan

Before:

Screen.Recording.2025-12-09.at.09.15.57.mov

After:

Screen.Recording.2025-12-09.at.09.16.45.mov

Integration tests in SharedRendererTests.ts
Unit tests for DECRQM response
Manual testing with escape sequences in demo
All existing tests pass
Lint passes

Manual Testing

# Lines appear atomically after delay
printf '\e[?2026h'; for i in 1 2 3 4 5; do echo "Line $i"; sleep 0.2; done; printf '\e[?2026l'

# Feature detection - should respond with CSI ? 2026 ; 2 $ y (supported, inactive)
printf '\e[?2026$p'

Closes #3375

🤖 Generated with Claude Code

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 xtermjs#3375 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <[email protected]>

Tyriar

How are you confirming the feature?

Bunch of CI failues

src/browser/services/RenderService.ts

src/common/InputHandler.ts

src/common/services/OptionsService.ts

Tyriar · 2025-12-09T14:35:39Z

src/browser/services/RenderService.ts

+        // First request in this sync cycle
+        this._synchronizedOutputStart = start;
+        this._synchronizedOutputEnd = end;


This looks very similar to DirtyRowTracker, pulling that out of InputHandler and using it would reduce some repetition.

Looked into this - the shared logic is just the min/max row tracking (~4 lines). DirtyRowTracker has different reset semantics (resets to cursor position via IBufferService) and doesn't need timeout management. Felt like an abstraction would add complexity without much benefit, but happy to revisit if you feel strongly.

src/browser/services/RenderService.ts

src/browser/services/RenderService.test.ts

typings/xterm.d.ts

chrislloyd · 2025-12-09T15:00:53Z

Thanks for the quick review!

How are you confirming the feature?

Works in the example project - will record a video and put it up.

- Remove synchronizedOutputTimeout public API, hardcode 1s timeout - Extract SynchronizedOutputHandler class for cleaner code - Update spec URL to contour-terminal/vt-extensions - Remove unnecessary comment in InputHandler - Delete unit tests, add integration tests in SharedRendererTests - Fix whitespace issue 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <[email protected]>

🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <[email protected]>

Applications can now detect synchronized output support by querying CSI ? 2026 $ p and receiving CSI ? 2026 ; <mode> $ y response. 🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <[email protected]>

chrislloyd · 2025-12-09T17:26:33Z

Updated with videos in the description.

Tyriar · 2025-12-10T15:58:02Z

Posix test script:

clear;c=$(tput cols);r=$(tput lines);for f in $(seq 0 10 200); do printf '\e[?2026h\e[H';for y in $(seq 0 $((r-1)));do for x in $(seq 0 $((c-1)));do printf "\e[48;5;$((16+(x+y+f)%216))m ";done;echo;done;printf '\e[?2026l';sleep 0.03;done

Tyriar

Tests should pass now, thanks @chrislloyd! I think for VS Code we've frozen builds for the holidays but I expect this to be in Insiders shortly after New Year's.

Tyriar · 2025-12-20T14:09:56Z

Almost, fixing 😅

chrislloyd · 2026-01-12T17:37:05Z

@Tyriar amazing, thank you so much ❤️ !!

chrislloyd mentioned this pull request Dec 6, 2025

Synchronized Output #3375

Closed

Tyriar requested changes Dec 9, 2025

View reviewed changes

chrislloyd and others added 3 commits December 9, 2025 07:58

Fix spec URL to use master branch

5d0eae8

🤖 Generated with [Claude Code](https://claude.com/claude-code) Co-Authored-By: Claude <[email protected]>

chrislloyd requested a review from Tyriar December 9, 2025 17:33

Tyriar added 3 commits December 10, 2025 07:49

Add BSU and ESU to VT tab in demo

7f81df5

Prefer const enum so the number is inlined

7cdf3f9

Move SynchronizedOutputHandler to bottom of file

9ea059d

Tyriar added 2 commits December 10, 2025 07:59

Fix lint

bd361af

Fix one playwright test

7f50b56

Tyriar self-assigned this Dec 20, 2025

Tyriar added this to the 6.0.0 milestone Dec 20, 2025

Tyriar added 2 commits December 20, 2025 06:04

Fix colors and clear cached frames to pass tests

832aa54

Add synchronizedOutputMode to xterm-headless.d.ts

32553ff

Tyriar approved these changes Dec 20, 2025

View reviewed changes

Tyriar enabled auto-merge December 20, 2025 14:07

Tyriar added 7 commits December 20, 2025 06:14

Add synchronizedOutput mode to headless terminal object

1eee5cd

Add node about serialize addon support

2edd121

Add missing property to headless API test assertion

ed0ab5f

Explicitly set background color in playwright tests

e9ba31f

Use underline cursor style to ensure cursor color isn't reported

12c4f95

Revert last frame request cache clearing

ad07f2c

Invert initial assertion

6733e13

Tyriar merged commit cb4d1ef into xtermjs:master Dec 20, 2025
19 of 22 checks passed

jasonftl mentioned this pull request Jan 9, 2026

[BUG] In-progress Call causes Screen Flickering anthropics/claude-code#769

Open

4 tasks

sgeraldes mentioned this pull request Jan 22, 2026

[Bug]: TUI animations scroll instead of animating in-place (missing xterm.js Synchronized Output support) wavetermdev/waveterm#2787

Open

Tyriar mentioned this pull request Feb 2, 2026

[Terminal] Scrollbar Abnormal Jumping: Page Shaking and Scrollbar Locked at Top When Using AI CLI Tools with Long Outputs #5620

Closed

Axelj00 mentioned this pull request Mar 22, 2026

Terminal scrolls to top of scrollback while waiting for agent response (10th iteration) clawterm/clawterm#227

Closed

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chrislloyd commented Dec 4, 2025 • edited Loading Uh oh! There was an error while loading. Please reload this page.

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Summary

Features

Test Plan

Manual Testing

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chrislloyd commented Dec 9, 2025

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chrislloyd commented Dec 4, 2025 •

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