feat(tui): added viewport-only transcript tail rendering and deferred resize repaint

- Added tail-only transcript rendering via `renderViewportTail`, stopping at `maxRows` and returning `EMPTY_TAIL`.
- Added `ViewportTailProvider`/`asViewportTailProvider` API and resize getters for viewport state.
- Implemented viewport-only painting during non-mux resize with deferred full repaint after settle.
- Added settle-resize helpers and coverage for deferred repaint timing and cancel paths.

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

@@ -4,6 +4,7 @@ import {
 	type NativeScrollbackCommittedRows,
 	type NativeScrollbackLiveRegion,
 	type RenderStablePrefix,
+	type ViewportTailProvider,
 } from "@oh-my-pi/pi-tui";
 
 const kSnapshot = Symbol("transcript.liveDiffSnapshot");
@@ -139,6 +140,8 @@ interface BlockSegment {
 }
 
 const EMPTY_SEGMENTS: BlockSegment[] = [];
+/** Shared empty result for an empty viewport-tail render (no allocation). */
+const EMPTY_TAIL: readonly string[] = [];
 
 interface LiveCommitState {
 	appendOnly: boolean;
@@ -415,7 +418,7 @@ function deriveLiveCommitState(
  */
 export class TranscriptContainer
 	extends Container
-	implements NativeScrollbackLiveRegion, NativeScrollbackCommittedRows, RenderStablePrefix
+	implements NativeScrollbackLiveRegion, NativeScrollbackCommittedRows, RenderStablePrefix, ViewportTailProvider
 {
 	// Bumped to retire every block's diff snapshot at once (theme change /
 	// clear); a snapshot is only honored when its stored generation matches.
@@ -520,6 +523,49 @@ export class TranscriptContainer
 		return index === children.length - 1;
 	}
 
+	/**
+	 * Render only the bottom `maxRows` rows of the transcript at `width`, walking
+	 * blocks from the last toward the first and stopping the instant enough rows
+	 * are collected — blocks above the fold are never rendered. The engine's
+	 * resize viewport fast path uses this so a drag (a SIGWINCH burst, each event
+	 * a fresh width that misses every per-width cache) re-lays-out only the
+	 * handful of visible blocks instead of the whole history every event.
+	 *
+	 * State-isolated by contract: touches none of the persistent full-compose
+	 * fields (#lines, #segments, the per-block diff snapshots, the commit/stable
+	 * bookkeeping), so the authoritative full render on settle reconciles exactly
+	 * as if this never ran. Calling each block's render() still warms its own
+	 * per-width cache, which that settle render then reuses for free.
+	 *
+	 * Consecutive visible blocks are joined by exactly one blank separator, the
+	 * same rule render() applies, so the result equals the bottom of a full
+	 * render except for an at-most-one-row separator on the topmost included
+	 * block — a transient discrepancy the settle paint overwrites.
+	 */
+	renderViewportTail(width: number, maxRows: number): readonly string[] {
+		width = Math.max(1, width);
+		if (maxRows <= 0) return EMPTY_TAIL;
+		const collected: (readonly string[])[] = [];
+		let total = 0;
+		for (let i = this.children.length - 1; i >= 0 && total < maxRows; i--) {
+			const contribution = stripPlainBlankEdges(this.children[i]!.render(width));
+			if (contribution.length === 0) continue;
+			// One blank separator sits between this block and the (already
+			// collected) visible block below it.
+			if (collected.length > 0) total += 1;
+			collected.push(contribution);
+			total += contribution.length;
+		}
+		if (collected.length === 0) return EMPTY_TAIL;
+		const rows: string[] = [];
+		for (let k = collected.length - 1; k >= 0; k--) {
+			if (rows.length > 0) rows.push("");
+			const body = collected[k]!;
+			for (let j = 0; j < body.length; j++) rows.push(body[j]!);
+		}
+		return rows.length > maxRows ? rows.slice(rows.length - maxRows) : rows;
+	}
+
 	override render(width: number): readonly string[] {
 		width = Math.max(1, width);
 		this.#nativeScrollbackLiveRegionStart = undefined;