diff --git a/packages/tui/src/tui.ts b/packages/tui/src/tui.ts
index 3fabe53dd7c..92d07f5dd2e 100644
--- a/packages/tui/src/tui.ts
+++ b/packages/tui/src/tui.ts
@@ -899,6 +899,15 @@ export class TUI extends Container {
 
 		newLines = this.applyLineResets(newLines);
 
+		// Sync render state for early returns that don't move the viewport.
+		const commitState = (): void => {
+			this.positionHardwareCursor(cursorPos, newLines.length);
+			this.previousLines = newLines;
+			this.previousWidth = width;
+			this.previousHeight = height;
+			this.previousViewportTop = prevViewportTop;
+		};
+
 		// Helper to clear scrollback and viewport and render all new lines
 		const fullRender = (clear: boolean): void => {
 			this.fullRedrawCount += 1;
@@ -1035,11 +1044,15 @@ export class TUI extends Container {
 				this.cursorRow = targetRow;
 				this.hardwareCursorRow = targetRow;
 			}
-			this.positionHardwareCursor(cursorPos, newLines.length);
-			this.previousLines = newLines;
-			this.previousWidth = width;
-			this.previousHeight = height;
-			this.previousViewportTop = prevViewportTop;
+			commitState();
+			return;
+		}
+
+		// All changes are strictly above the visible viewport (e.g. a spinner ticking
+		// offscreen while a tall ui.custom dialog fills the screen). The terminal
+		// already shows the correct pixels, so just sync state without writing.
+		if (lastChanged < prevViewportTop && newLines.length === this.previousLines.length) {
+			commitState();
 			return;
 		}
 
diff --git a/packages/tui/test/regression-offscreen-change-full-redraw.test.ts b/packages/tui/test/regression-offscreen-change-full-redraw.test.ts
new file mode 100644
index 00000000000..8e725fb6b92
--- /dev/null
+++ b/packages/tui/test/regression-offscreen-change-full-redraw.test.ts
@@ -0,0 +1,62 @@
+import assert from "node:assert";
+import { describe, it } from "node:test";
+import { type Component, TUI } from "../src/tui.js";
+import { VirtualTerminal } from "./virtual-terminal.js";
+
+/**
+ * Regression: when content is taller than the terminal and a component above
+ * the visible viewport changes (e.g. the Loader spinner ticking while a tall
+ * ui.custom dialog fills the screen), doRender() hit the
+ * `firstChanged < prevViewportTop` branch and did a fullRender(true) on every
+ * frame — manifesting as the screen "scrolling like crazy".
+ */
+
+class Lines implements Component {
+	constructor(public lines: string[]) {}
+	render(): string[] {
+		return this.lines;
+	}
+	invalidate() {}
+}
+
+const tick = () => new Promise<void>((r) => process.nextTick(r));
+
+describe("TUI differential render with offscreen changes", () => {
+	it("does not full-redraw when the only change is above the viewport", async () => {
+		const rows = 10;
+		const terminal = new VirtualTerminal(80, rows);
+		const tui = new TUI(terminal);
+
+		// chat + spinner are pushed above the viewport by a tall dialog
+		const spinner = new Lines(["⠋ Working"]);
+		const dialog = new Lines(Array.from({ length: rows + 2 }, (_, i) => `dialog ${i}`));
+		tui.addChild(new Lines(["chat 0", "chat 1", "chat 2"]));
+		tui.addChild(spinner);
+		tui.addChild(dialog);
+
+		tui.start();
+		await tick();
+		await terminal.flush();
+
+		const initialViewport = terminal.getViewport();
+		assert.ok(!initialViewport.some((l) => l.includes("Working")), "precondition: spinner is offscreen");
+		assert.ok(
+			initialViewport.some((l) => l.includes(`dialog ${rows + 1}`)),
+			"precondition: dialog bottom visible",
+		);
+
+		const redrawsBefore = tui.fullRedraws;
+
+		for (const f of ["⠙", "⠹", "⠸", "⠼", "⠴"]) {
+			spinner.lines = [`${f} Working`];
+			tui.requestRender();
+			await tick();
+		}
+		await terminal.flush();
+
+		assert.strictEqual(tui.fullRedraws, redrawsBefore, "offscreen-only change must not trigger fullRender");
+		assert.deepStrictEqual(terminal.getViewport(), initialViewport, "viewport must remain stable");
+
+		tui.stop();
+	});
+});