diff --git a/packages/pi-tui/src/ledger/engine.ts b/packages/pi-tui/src/ledger/engine.ts
index ca50b56c3..baf60c9a8 100644
--- a/packages/pi-tui/src/ledger/engine.ts
+++ b/packages/pi-tui/src/ledger/engine.ts
@@ -7,6 +7,8 @@ import { getNativeScrollbackCommitSafeEnd, getNativeScrollbackLiveRegionStart, g
 import { coalesceAdjacentSgr } from "./sgr-coalesce.ts";
 import {
 	type CursorControlResult,
+	ALT_SCREEN_ENTER,
+	ALT_SCREEN_EXIT,
 	DISABLE_AUTOWRAP,
 	ENABLE_AUTOWRAP,
 	ERASE_LINE,
@@ -53,7 +55,6 @@ export class LedgerTuiEngine {
 	// ---- resize viewport defer (Phase B Task 4) ----
 	#resizeViewportActive = false;
 	#resizeViewportSettleTimer: ReturnType<typeof setTimeout> | undefined;
-	#resizeAltActive = false;
 	#resizeViewportPaintCount = 0;
 	static readonly #RESIZE_SETTLE_MS = 120;
 
@@ -700,9 +701,8 @@ export class LedgerTuiEngine {
 	public beginResizeViewport(): void {
 		if (isMultiplexerSession()) return; // mux: no alt-screen
 		if (!this.#resizeViewportActive) {
-			this.terminal.write("\x1b[?1049h");
+			this.terminal.write(ALT_SCREEN_ENTER);
 			this.#resizeViewportActive = true;
-			this.#resizeAltActive = true;
 		}
 		this.#armResizeSettle();
 	}
@@ -717,8 +717,7 @@ export class LedgerTuiEngine {
 		this.#resizeViewportSettleTimer = undefined;
 		if (this.#resizeViewportActive) {
 			this.#resizeViewportActive = false;
-			this.#resizeAltActive = false;
-			this.terminal.write("\x1b[?1049l");
+			this.terminal.write(ALT_SCREEN_EXIT);
 		}
 		// authoritative full repaint after settle (clear scrollback, rewrap)
 		this.requestFullPaint(true);
@@ -746,15 +745,23 @@ export class LedgerTuiEngine {
 			clearTimeout(this.#resizeViewportSettleTimer);
 			this.#resizeViewportSettleTimer = undefined;
 		}
-		if (this.#resizeAltActive) {
+		if (this.#resizeViewportActive) {
 			this.#resizeViewportActive = false;
-			this.#resizeAltActive = false;
-			this.terminal.write("\x1b[?1049l");
+			this.terminal.write(ALT_SCREEN_EXIT);
 		}
 	}
 
 	public reset(): void {
 		this.#stopped = false;
+		if (this.#resizeViewportSettleTimer) {
+			clearTimeout(this.#resizeViewportSettleTimer);
+			this.#resizeViewportSettleTimer = undefined;
+		}
+		if (this.#resizeViewportActive) {
+			this.#resizeViewportActive = false;
+			this.terminal.write(ALT_SCREEN_EXIT);
+		}
+		this.#resizeViewportPaintCount = 0;
 		this.#committedRows = 0;
 		this.#committedPrefix = [];
 		this.#committedPrefixAuditRows = 0;
diff --git a/packages/pi-tui/test/resize-defer.test.ts b/packages/pi-tui/test/resize-defer.test.ts
index ccac577ff..065b0523d 100644
--- a/packages/pi-tui/test/resize-defer.test.ts
+++ b/packages/pi-tui/test/resize-defer.test.ts
@@ -16,8 +16,9 @@ const sleep = (ms: number) => new Promise<void>((resolve) => setTimeout(resolve,
 // Env vars that make isMultiplexerSession() return true. The non-mux fast path is
 // gated on `!isMultiplexerSession()`, so a non-mux test must clear these — the
 // developer/CI shell often runs inside tmux/cmux, which would otherwise force the
-// mux path and make the assertions vacuous.
-const MUX_ENV_KEYS = ["TMUX", "STY", "ZELLIJ", "CMUX_WORKSPACE_ID", "CMUX_SURFACE_ID"] as const;
+// mux path and make the assertions vacuous. TERM is included because tmux/screen
+// set it to `tmux-256color`/`screen-256color`, which alone flips isMultiplexerSession().
+const MUX_ENV_KEYS = ["TMUX", "STY", "ZELLIJ", "CMUX_WORKSPACE_ID", "CMUX_SURFACE_ID", "TERM"] as const;
 
 function snapshotMuxEnv(): Record<string, string | undefined> {
 	const snap: Record<string, string | undefined> = {};
@@ -67,6 +68,9 @@ async function withLedgerMux<T>(run: () => Promise<T>): Promise<T> {
 const ALT_ENTER = "\x1b[?1049h";
 const ALT_EXIT = "\x1b[?1049l";
 const ED3 = "\x1b[3J";
+// Per-row erase emitted only by the resize viewport fast path (#renderResizeViewport);
+// the full/update paint paths use ED2/ED3 or ERASE_TO_END_OF_LINE (\x1b[K), never \x1b[2K.
+const ERASE_LINE = "\x1b[2K";
 
 describe("ledger resize viewport defer", () => {
 	it("uses the alt-screen fast path during a resize drag (no ED3)", async () => {
@@ -89,6 +93,10 @@ describe("ledger resize viewport defer", () => {
 
 			const writes = terminal.getWrites();
 			assert.ok(writes.includes(ALT_ENTER), `drag should enter the alt-screen; got: ${JSON.stringify(writes)}`);
+			assert.ok(
+				writes.includes(ERASE_LINE),
+				`drag should paint via the viewport fast path (per-row \\x1b[2K); got: ${JSON.stringify(writes)}`,
+			);
 			assert.ok(!writes.includes(ED3), `drag must not clear scrollback (ED3); got: ${JSON.stringify(writes)}`);
 			tui.stop();
 		});