fix(tui): compact transcript retention without losing resize history

[RensTillmann]

Summary

• compact finalized transcript head rows after they enter native scrollback, releasing retained render arrays in long sessions
• rehydrate compacted history before destructive resize/reset full paints through a new NativeScrollbackReplay contract
• keep versioned/post-finalize-mutable blocks out of compaction and preserve committed-block bookkeeping
• bound the Markdown L2 render cache to 512 KiB total and 32 KiB per entry

Fixes #4820.

Context

This builds on the approach in #4841 by @cexll. That PR's review identified a P1 data-loss case: after compaction, a non-multiplexer resize or reset clears native scrollback but the compacted prefix cannot be replayed. This version adds an explicit pre-full-paint replay hook, so TranscriptContainer re-renders the complete child history for that paint and resumes compaction on the following frame.

Tests

• bun test packages/tui/test/component-render.test.ts packages/coding-agent/test/modes/components/transcript-container.test.ts packages/tui/test/markdown.test.ts — 170 passed
• bun run check:types in packages/tui — passed
• bun run check:types in packages/coding-agent — passed
• Biome check on all touched source/test files — passed
• standalone binary build, --version, --help, and omp read /etc/hostname smoke tests — passed

Regression coverage

• committed finalized head rows are dropped from subsequent frames
• compacted rows remain classified as committed
• destructive full paints call the replay hook before composition
• replay survives the engine feeding its previous committed-row count before render
• oversized Markdown renders are excluded from the shared L2 cache

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[tcf909]

Follow-up fix for a live crash exposed by this PR's compaction path:

TypeError: undefined is not an object (evaluating 'segment.component') in TranscriptContainer.isBlockUncommitted when #retireIrcCard walks sparse compacted #segments holes after a later render.

PR: #5298

• Guards undefined segment slots
• Red/green regression for the sparse-hole path

[tcf909]

Crash follow-up for this PR

Live crash after applying this PR:

TypeError: undefined is not an object (evaluating 'segment.component')
  at isBlockUncommitted
  at #retireIrcCard

Cause

#compactCommittedPrefix leaves zero-row placeholders, then a later render() allocates new Array(count) and only fills from #compactedChildStart, so compacted indices become sparse undefined holes. isBlockUncommitted() iterated every segment and crashed.

Fix

Guard undefined segment slots + red/green regression.

Verification

• Red without guard: test fails with original TypeError
• Green with guard: bun test packages/coding-agent/test/modes/components/transcript-container.test.ts → 48 pass / 0 fail
• Focused install-tree suite for empty-stop + mixed-assistant + container: 59 pass / 0 fail

I opened #5298 but it was auto-closed (author not vouched). Please cherry-pick this onto the PR branch:

From c14513a8b03c26d989c78da68484302be926d7df Mon Sep 17 00:00:00 2001
From: "T.C. Ferguson" <tc.ferguson@forwardthinking.com>
Date: Sun, 12 Jul 2026 09:52:39 -1000
Subject: [PATCH] fix(tui): skip sparse compacted segments in
 isBlockUncommitted

After #5239 compaction, a later render only fills segments from
#compactedChildStart, leaving undefined holes in the prefix. Iterating
those entries crashed when retiring IRC/ephemeral cards
(`segment2.component`). Guard undefined segment slots and cover the
sparse-hole path with a red/green regression.
---
 .../modes/components/transcript-container.ts  |  6 +++++-
 .../components/transcript-container.test.ts   | 20 +++++++++++++++++++
 2 files changed, 25 insertions(+), 1 deletion(-)

diff --git a/packages/coding-agent/src/modes/components/transcript-container.ts b/packages/coding-agent/src/modes/components/transcript-container.ts
index 428b46831..576dec064 100644
--- a/packages/coding-agent/src/modes/components/transcript-container.ts
+++ b/packages/coding-agent/src/modes/components/transcript-container.ts
@@ -242,9 +242,13 @@ export class TranscriptContainer
 	 */
 	isBlockUncommitted(component: Component): boolean {
 		const index = this.children.indexOf(component);
+		// Compacted prefix is already committed native history and must not be
+		// retracted. Compacted slots may be sparse holes after a later re-render
+		// (render only fills from #compactedChildStart), so the loop below must
+		// skip undefined entries.
 		if (index >= 0 && index < this.#compactedChildStart) return false;
 		for (const segment of this.#segments) {
-			if (segment.component !== component) continue;
+			if (segment === undefined || segment.component !== component) continue;
 			return segment.rowCount === 0 || segment.startRow >= this.#committedRows;
 		}
 		return true;
diff --git a/packages/coding-agent/test/modes/components/transcript-container.test.ts b/packages/coding-agent/test/modes/components/transcript-container.test.ts
index 56b24164c..6a812f0ba 100644
--- a/packages/coding-agent/test/modes/components/transcript-container.test.ts
+++ b/packages/coding-agent/test/modes/components/transcript-container.test.ts
@@ -700,6 +700,26 @@ describe("TranscriptContainer isBlockUncommitted", () => {
 		container.setNativeScrollbackCommittedRows(100);
 		expect(container.isBlockUncommitted(empty)).toBe(true);
 	});
+
+	it("survives sparse compacted segment holes when checking uncommitted status", () => {
+		const container = new TranscriptContainer();
+		const committed = new StreamingBlock(["committed"], true);
+		const live = new StreamingBlock(["live"], true);
+		container.addChild(committed);
+		container.addChild(live);
+
+		expect(container.render(40)).toEqual(["committed", "", "live"]);
+		container.setNativeScrollbackCommittedRows(2);
+		// Compaction first rewrites the compacted prefix into zero-row placeholders.
+		expect(container.render(40)).toEqual(["live"]);
+		// The next render only repopulates from #compactedChildStart, leaving a
+		// sparse hole at the compacted index. Retiring IRC/ephemeral cards walks
+		// every segment and must not crash on those undefined entries.
+		expect(container.render(40)).toEqual(["live"]);
+		expect(() => container.isBlockUncommitted(live)).not.toThrow();
+		expect(() => container.isBlockUncommitted(committed)).not.toThrow();
+		expect(container.isBlockUncommitted(committed)).toBe(false);
+	});
 });
 
 describe("TranscriptContainer renderViewportTail", () => {
-- 
2.50.1 (Apple Git-155)