fix(coding-agent): fixed append-only detection for in-place streaming line growth

- Updated live transcript state derivation to treat a trailing-line extension with unchanged rows as append-only.
- Replaced the prior append-grew heuristic with preserved-row checks so non-rewritten history is not marked volatile.
- Added a regression test covering in-place streaming line growth and scrollback commit safety in transcript containers.

packages/coding-agent/test/transcript-streaming-commit-repro.test.ts

@@ -0,0 +1,52 @@
+import { describe, expect, it } from "bun:test";
+import { type Component, TERMINAL } from "@oh-my-pi/pi-tui";
+import { TranscriptContainer } from "../src/modes/components/transcript-container";
+
+type MutableTerminalInfo = { eagerEraseScrollbackRisk: boolean };
+const mutableTerminalInfo = TERMINAL as unknown as MutableTerminalInfo;
+
+async function withTerminalRisk<T>(risk: boolean, run: () => T | Promise<T>): Promise<T> {
+	const saved = TERMINAL.eagerEraseScrollbackRisk;
+	mutableTerminalInfo.eagerEraseScrollbackRisk = risk;
+	try {
+		return await run();
+	} finally {
+		mutableTerminalInfo.eagerEraseScrollbackRisk = saved;
+	}
+}
+
+class MutableLiveBlock implements Component {
+	#lines: string[];
+	constructor(lines: string[]) {
+		this.#lines = [...lines];
+	}
+	render(width: number): string[] {
+		return this.#lines.map(line => line.slice(0, width));
+	}
+	setLines(lines: string[]): void {
+		this.#lines = [...lines];
+	}
+	isTranscriptBlockFinalized(): boolean {
+		return false;
+	}
+}
+
+describe("transcript streaming commit (assistant text)", () => {
+	it("treats in-place growth of the trailing line as append-only", async () => {
+		await withTerminalRisk(true, () => {
+			const chat = new TranscriptContainer();
+			// Models a streaming assistant reply: stable head rows plus a current
+			// line that grows token-by-token without adding a new row.
+			const block = new MutableLiveBlock(["para one", "para two", "the quick brown"]);
+			chat.addChild(block);
+
+			chat.render(80);
+
+			block.setLines(["para one", "para two", "the quick brown fox"]);
+			chat.render(80);
+			// The head rows never changed; only the trailing line grew. Its scrolled-
+			// off head must be committable to native scrollback (tmux pane history).
+			expect(chat.getNativeScrollbackCommitSafeEnd()).toBe(3);
+		});
+	});
+});