fix(tui): validate leading width epoch blocks

Amp-Thread-ID: https://ampcode.com/threads/T-019ff125-f53d-739b-8462-6027fe70f914
Co-authored-by: Amp <amp@ampcode.com>

packages/coding-agent/src/modes/components/transcript-container.ts

@@ -191,6 +191,7 @@ export class TranscriptContainer
 			segment: BlockSegment;
 			childBoundary: unknown;
 			childHasBoundary: boolean;
+			precedingSegments: BlockSegment[];
 			trailingSegments: BlockSegment[];
 		}
 	>();
@@ -265,12 +266,14 @@ export class TranscriptContainer
 			typeof child.captureNativeScrollbackWidthEpoch === "function" &&
 			typeof child.resolveNativeScrollbackWidthEpoch === "function" &&
 			typeof child.getNativeScrollbackWidthEpochRows === "function";
+		const segmentIndex = this.#segments.indexOf(segment);
 		const marker = {};
 		this.#widthEpochBoundaries.set(marker, {
 			segment,
 			childBoundary: childHasBoundary ? child.captureNativeScrollbackWidthEpoch?.() : undefined,
 			childHasBoundary,
-			trailingSegments: this.#segments.slice(this.#segments.indexOf(segment) + 1),
+			precedingSegments: this.#segments.slice(0, segmentIndex),
+			trailingSegments: this.#segments.slice(segmentIndex + 1),
 		});
 		return marker;
 	}
@@ -279,8 +282,26 @@ export class TranscriptContainer
 		if (typeof boundary !== "object" || boundary === null) return undefined;
 		const marker = this.#widthEpochBoundaries.get(boundary);
 		if (!marker) return undefined;
-		const current = this.#segments.find(segment => segment.component === marker.segment.component);
+		const currentIndex = this.#segments.findIndex(segment => segment.component === marker.segment.component);
+		const current = this.#segments[currentIndex];
 		if (!current) return undefined;
+		if (currentIndex !== marker.precedingSegments.length) return undefined;
+		for (let i = 0; i < marker.precedingSegments.length; i++) {
+			const captured = marker.precedingSegments[i]!;
+			const preceding = this.#segments[i]!;
+			// A width-dependent physical row count cannot distinguish ordinary
+			// reflow from logical growth. Without a mutation version the leading
+			// boundary is unverifiable, so replay the epoch conservatively.
+			if (
+				preceding.component !== captured.component ||
+				!captured.finalized ||
+				!preceding.finalized ||
+				captured.version === undefined ||
+				preceding.version !== captured.version
+			) {
+				return undefined;
+			}
+		}
 		if (!marker.childHasBoundary) {
 			if (marker.segment.rowCount === 0) return current.startRow;
 			if (!marker.segment.finalized) return undefined;

packages/coding-agent/test/modes/components/transcript-container.test.ts

@@ -78,6 +78,20 @@ class DeclaredSettledStreamingBlock extends StreamingBlock {
 	}
 }
 
+class WidthEpochStreamingBlock extends DeclaredSettledStreamingBlock {
+	captureNativeScrollbackWidthEpoch(): unknown {
+		return {};
+	}
+
+	resolveNativeScrollbackWidthEpoch(_boundary: unknown): number | undefined {
+		return 1;
+	}
+
+	getNativeScrollbackWidthEpochRows(): number | undefined {
+		return 1;
+	}
+}
+
 class CountingFinalizedBlock implements Component {
 	renderCount = 0;
 	#lines: string[];
@@ -369,6 +383,38 @@ describe("TranscriptContainer", () => {
 		expect(rendered.at(-1)).toContain("Finalized notice");
 	});
 
+	it("replays conservatively when finalized history before a live source has no mutation version", () => {
+		const container = new TranscriptContainer();
+		const history = new CountingFinalizedBlock(["history"]);
+		const live = new WidthEpochStreamingBlock(["stable", "pending"], 1);
+		container.addChild(history);
+		container.addChild(live);
+		container.render(40);
+		const boundary = container.captureNativeScrollbackWidthEpoch();
+
+		history.set(["history", "late image"]);
+		container.render(17);
+
+		expect(container.resolveNativeScrollbackWidthEpoch(boundary)).toBeUndefined();
+	});
+
+	it("rejects a width epoch when versioned finalized history before its live source grows", () => {
+		const container = new TranscriptContainer();
+		const history = new VersionedFinalizedBlock(["history"]);
+		const live = new WidthEpochStreamingBlock(["stable", "pending"], 1);
+		container.addChild(history);
+		container.addChild(live);
+		container.render(40);
+		const boundary = container.captureNativeScrollbackWidthEpoch();
+		container.render(17);
+		expect(container.resolveNativeScrollbackWidthEpoch(boundary)).toBeGreaterThan(0);
+
+		history.mutate(["history", "late image"]);
+		container.render(17);
+
+		expect(container.resolveNativeScrollbackWidthEpoch(boundary)).toBeUndefined();
+	});
+
 	it("rejects a captured live tail that mutates while finalizing", () => {
 		const container = new TranscriptContainer();
 		const assistant = new AssistantMessageComponent();