perf(agent-manager): render terminal output with WebGL and pause hidden terminals

[marius-kilocode]

Streaming terminal output in Agent Manager (background setup scripts, Run/Setup flows) was expensive in two independent ways. The DOM renderer creates one element per cell per row, so sustained output dominated main-thread work with per-cell churn and a layout/style pass per render cycle. Terminals hidden in the background had no sharing benefit either: visibility was toggled with opacity only, so hidden terminals kept their render loop running at full rate for output nobody sees.

This change makes each of those parts cheap:

• Terminals render with the WebGL renderer, painting the whole viewport in one draw instead of per-cell elements. Falls back to the DOM renderer when WebGL2 is unavailable.
• PTY chunks are coalesced into one xterm write per animation frame instead of one parse + render schedule per WebSocket message, with a byte cap and watchdog so the batch stays bounded when frames stall.
• Inactive terminal slots now use display: none. xterm 6's render service observes the screen element, pauses hidden terminals (rAF loop, model updates, GPU draws), and replays a full refresh on reactivation; the existing fit + refresh activation repaint stays as insurance.

Measured on a 7s streaming capture (isolated webview): layout duration 92 ms to ~0 ms, recalc-style 122 ms to ~0 ms, layout count 262 to 2; hidden-terminal streaming dropped from an identical full-rate render pipeline to parser work only. No bundle size change.

Includes the batcher unit tests and the changeset.

[kilo-code-bot]

Code Review Summary

Status: No Issues Found | Recommendation: Merge

Files Reviewed (4 files)

• packages/kilo-vscode/webview-ui/agent-manager/agent-manager.css
• packages/kilo-vscode/webview-ui/agent-manager/terminal/SideTerminalPanel.tsx
• packages/kilo-vscode/webview-ui/agent-manager/terminal/TerminalTab.tsx
• packages/kilo-vscode/webview-ui/agent-manager/terminal/render.tsx

Previous Review Summaries (2 snapshots, latest commit 80024c0)

Current summary above is authoritative. Previous snapshots are kept for context only.

Previous review (commit 80024c0)

Status: 3 Issues Found | Recommendation: Address before merge

Overview

Severity	Count
CRITICAL	0
WARNING	3
SUGGESTION	0

Issue Details (click to expand)

WARNING

File	Line	Issue
packages/kilo-vscode/webview-ui/agent-manager/terminal/TerminalTab.tsx	177	WebGL context per never-unmounted terminal, no context-loss fallback
packages/kilo-vscode/webview-ui/agent-manager/terminal/TerminalTab.tsx	309	Status term.writeln calls bypass the new write batcher
packages/kilo-vscode/webview-ui/agent-manager/agent-manager.css	4938	display: none drops layout size so hidden PTY output wraps at the wrong cols

Files Reviewed (9 files)

• .changeset/agent-manager-terminal-render-performance.md - 0 issues
• bun.lock - 0 issues
• packages/kilo-vscode/package.json - 0 issues
• packages/kilo-vscode/tests/unit/agent-manager-terminal-replay.test.ts - 0 issues
• packages/kilo-vscode/webview-ui/agent-manager/agent-manager.css - 1 issue
• packages/kilo-vscode/webview-ui/agent-manager/terminal/SideTerminalPanel.tsx - 0 issues
• packages/kilo-vscode/webview-ui/agent-manager/terminal/TerminalTab.tsx - 2 issues
• packages/kilo-vscode/webview-ui/agent-manager/terminal/render.tsx - 0 issues
• packages/kilo-vscode/webview-ui/agent-manager/terminal/replay.ts - 0 issues

Fix these issues in Kilo Cloud

Previous review

Status: 2 Issues Found | Recommendation: Address before merge

Overview

Severity	Count
CRITICAL	0
WARNING	2
SUGGESTION	0

Issue Details (click to expand)

WARNING

File	Line	Issue
packages/kilo-vscode/webview-ui/agent-manager/terminal/TerminalTab.tsx	177	WebGL context per never-unmounted terminal, no context-loss fallback
packages/kilo-vscode/webview-ui/agent-manager/agent-manager.css	4938	display: none drops layout size so hidden PTY output wraps at the wrong cols

Files Reviewed (9 files)

• .changeset/agent-manager-terminal-render-performance.md - 0 issues
• bun.lock - 0 issues
• packages/kilo-vscode/package.json - 0 issues
• packages/kilo-vscode/tests/unit/agent-manager-terminal-replay.test.ts - 0 issues
• packages/kilo-vscode/webview-ui/agent-manager/agent-manager.css - 1 issue
• packages/kilo-vscode/webview-ui/agent-manager/terminal/SideTerminalPanel.tsx - 0 issues
• packages/kilo-vscode/webview-ui/agent-manager/terminal/TerminalTab.tsx - 1 issue
• packages/kilo-vscode/webview-ui/agent-manager/terminal/render.tsx - 0 issues
• packages/kilo-vscode/webview-ui/agent-manager/terminal/replay.ts - 0 issues

Fix these issues in Kilo Cloud

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_{Reviewed by grok-4.6 · Input: 96.6K · Output: 15.4K · Cached: 551.2K}

_{Review guidance: REVIEW.md from base branch main}

[marius-kilocode]

Measured performance improvement

Measured on an isolated VS Code dev build with the self-test harness: 7.2s streaming capture (1500-line script output), webview performance profile.

Metric	Before (DOM renderer, opacity-only hidden)	After (WebGL + batching + render pause)	Improvement
Visible stream: LayoutDuration	92 ms	~0.7 ms	−99.2%
Visible stream: RecalcStyleDuration	122 ms	~0 ms	~100%
Visible stream: LayoutCount	262	2	−99.2%
Visible stream: ScriptDuration	132 ms	133 ms	0% (parse-bound, unchanged)
Hidden terminal: ScriptDuration	121 ms	84 ms	−30%
Hidden terminal: GPU render work	_updateModel, drawElementsInstanced, bufferData, clone	none (parser only)	100% eliminated
Page-trace totals (capture window)	script 912 ms, layout 287 ms, paint 146 ms	script 551 ms, layout 93 ms, paint 68 ms	−40% / −67% / −54%
Terminal switch cost	untested	4 ms JS, no >50 ms tasks	no long tasks

The visible-path win comes from removing browser layout/paint and per-cell DOM churn; the JS parse cost is unchanged because xterm must parse the output regardless. Background terminals now spend no render work on output nobody sees.