Add synchronized output support (DEC mode 2026)

[chrislloyd]

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

[Tyriar]

How are you confirming the feature?

Bunch of CI failues

[Tyriar]

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

[chrislloyd]

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.

[chrislloyd]

Thanks for the quick review!

How are you confirming the feature?

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

[chrislloyd]

Updated with videos in the description.

[Tyriar]

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