fix(tui): deferred bottom-anchored shrink on unknown posix viewports

A shrink across the viewport boundary on POSIX terminals that cannot report scrollback position (kitty, plain xterm) fell through to viewportRepaint. Repainting bottom-anchored newLines at newLength - height left rows newLength - height .. prevLength - height - 1 already committed to native scrollback, so they reappeared at the viewport top — two duplicated rows at the scrollback/viewport boundary in bjin's trace.\n\nMark scrollback dirty and emit deferredShrink instead (padding to the previous row count) so no native rows are re-emitted; the next checkpoint rebuild (e.g. prompt submit -> refreshNativeScrollbackIfDirty) cleans up.\n\nFixes #1566

packages/tui/src/tui.ts

@@ -1335,8 +1335,14 @@ export class TUI extends Container {
 			) {
 				return { kind: "historyRebuild" };
 			}
+			// POSIX terminals that cannot report viewport position fall through here
+			// (`canRebuildNativeScrollbackLive` is false): a viewport-only repaint would
+			// bottom-anchor `newLines` and re-emit the rows between the new and old
+			// viewport tops on top of the copies the terminal already kept in native
+			// scrollback. Pad to the previous row count instead and let the next
+			// checkpoint rebuild (e.g. prompt submit) clean up.
 			this.#markNativeScrollbackDirty();
-			return { kind: "viewportRepaint" };
+			return { kind: "deferredShrink", paddedLength: this.#previousLines.length };
 		}
 
 		const suppressSuffixScroll = this.#suppressNextSuffixScroll;

packages/tui/test/render-regressions.test.ts

@@ -1531,6 +1531,56 @@ describe("TUI terminal-state regressions", () => {
 				tui.stop();
 			}
 		});
+		it("defers bottom-anchored shrink when POSIX viewport state is unknown", async () => {
+			// Repro for #1566 follow-up (kitty/Linux): a bottom-anchored shrink across the
+			// viewport boundary used to fall through to `viewportRepaint`, which redrew the
+			// new transcript at `newLength - height` while leaving rows
+			// `[newLength - height .. prevLength - height - 1]` already in native
+			// scrollback — they reappeared at the top of the viewport, duplicating two rows
+			// at the boundary in the captured trace.
+			const term = new UnknownViewportTerminal(40, 6);
+			const tui = new TUI(term);
+			const body = rows("line-", 12);
+			const component = new MutableLinesComponent([...body, "spinner-row", "spacer-row", "prompt-row"]);
+			tui.addChild(component);
+
+			try {
+				tui.start();
+				await settle(term);
+
+				component.setLines([...body, "prompt-row"]);
+				tui.requestRender();
+				await settle(term);
+
+				const scrollback = term.getScrollBuffer();
+				for (let i = 0; i < body.length; i++) {
+					const pattern = new RegExp(`\\bline-${i}\\b`);
+					expect(
+						countMatches(scrollback, pattern),
+						`line-${i} must not duplicate at boundary`,
+					).toBeLessThanOrEqual(1);
+				}
+
+				expect(tui.refreshNativeScrollbackIfDirty({ allowUnknownViewport: true })).toBe(true);
+				await settle(term);
+				expect(visible(term).map(line => line.trim())).toEqual([
+					"line-7",
+					"line-8",
+					"line-9",
+					"line-10",
+					"line-11",
+					"prompt-row",
+				]);
+				const rebuilt = term.getScrollBuffer();
+				for (let i = 0; i < body.length; i++) {
+					const pattern = new RegExp(`\\bline-${i}\\b`);
+					expect(countMatches(rebuilt, pattern), `line-${i} appears once post-checkpoint`).toBe(1);
+				}
+			} finally {
+				tui.stop();
+			}
+		});
+
 		it("renders streaming row inserts on WSL Windows Terminal even when viewport probe is unavailable", async () => {
 			const originalPlatform = process.platform;
 			Object.defineProperty(process, "platform", { configurable: true, value: "linux" });