fix(webview): throttle state pushes to prevent gray screen OOM at high message counts

[edelauna]

Context

The chat webview turns gray (crashes) during long-running tasks. User-reported diagnostics from logWebviewHiddenDiagnostics confirm 3,525 messages at crash time. This is tracked upstream as a recurring user complaint — PR #153 reduced the react-virtuoso pre-render buffer but did not address the root cause.

Related: Part of the broader webview message architecture addressed by #355 (Stories 3.2c + 3.2d), but scoped as an independent fix that doesn't touch the message protocol or task lifecycle.

Developer Notes

Problem

addToClineMessages (Task.ts:1015) calls postStateToWebviewWithoutTaskHistory() on every single message addition, serializing and posting the entire clineMessages array to the webview. With 3,525 messages averaging ~2KB of tool output each, every state push is ~7MB. During active tool use with checkpoint saves, this fires dozens of times per minute. The webview must deserialize, merge into React state, and re-render each time — eventually exceeding the V8 heap limit and crashing the renderer (gray screen).

Secondary issue: aggregatedCostsMap in ChatView.tsx is never cleared on task change (line 480-491 cleanup effect), causing unbounded growth across task switches with subtasks.

Proposed Fix

Throttle the hot-path state pushes — no new message types, no protocol changes.

ClineProvider.ts — add throttled variant (~10 lines)

private _postStateToWebviewThrottled = debounce(
    () => this.postStateToWebviewWithoutTaskHistory(),
    500,
    { leading: true, trailing: true, maxWait: 1000 }
)

async postStateToWebviewThrottled(): Promise<void> {
    this._postStateToWebviewThrottled()
}

Task.ts — swap 2 hot-path call sites (2 line changes)

Line	Context	Change
1015	addToClineMessages — every new message	→ postStateToWebviewThrottled
527	messageQueueService.stateChanged	→ postStateToWebviewThrottled
1828	Task start (clear state)	Keep immediate
2551	Before API request	Keep immediate
3291	After streaming complete	Keep immediate

Partial message streaming is unaffected — updateClineMessage (Task.ts:1048) already uses the lightweight { type: "messageUpdated" } path, bypassing the full state push entirely.

ChatView.tsx:481 — clear aggregatedCostsMap on task change (1 line)

setAggregatedCostsMap(new Map())

Task.abortTask — flush debounce on abort (1 line)

Ensures the final state is delivered when a task is stopped.

Impact

At 3,525 messages:

• Before: ~7MB × dozens/sec during rapid tool use → OOM → gray screen
• After: ~7MB × max 2/sec (throttled) → webview stays alive

Risk Assessment

• Low risk: No new message types, no protocol changes, no task lifecycle changes
• UI delay: New message bubbles may appear up to 500ms late; streaming text is unaffected (uses messageUpdated path)
• Final state guaranteed: trailing: true + abort flush ensures last state is always delivered
• No fix(task-lifecycle): preserve parent-child link when delegated subtask is interrupted #560-style edge cases: This doesn't add states, transitions, or concurrency primitives — it just coalesces rapid calls to an existing method

Acceptance Criteria

• Chat remains responsive at 3,500+ messages (no gray screen)
• Streaming text still appears in real-time (messageUpdated path unchanged)
• Task start shows clean state immediately (line 1828 not throttled)
• Task abort delivers final state (debounce flushed)
• aggregatedCostsMap is empty after task switch
• Existing test suite passes unchanged