fix(tui): rebuilt blank deferred shrink from padded top

Codex review on #1599: the blank-viewport guard must compare the new transcript length against the viewport top used by the padded repaint, not #scrollbackHighWater. Prior unknown-POSIX viewport repaints can commit a long logical frame without advancing the high-water mark, so the stale high-water mark still lets all-blank deferred shrinks through.\n\nCompute paddedViewportTop from #previousLines.length - height and rebuild when the new tail cannot reach it. Add a regression for an offscreen POSIX viewport repaint that grows the committed frame to 120 rows while high-water remains at the original 20-row overflow, then shrinks to 15 rows.

packages/tui/src/tui.ts

@@ -1342,12 +1342,17 @@ export class TUI extends Container {
 			// previous row count so no committed row is re-emitted, and the next checkpoint
 			// rebuild (e.g. prompt submit -> `refreshNativeScrollbackIfDirty`) cleans up.
 			//
-			// That deferral only carries real content when `newLines.length > scrollbackHighWater`
-			// — otherwise the padded viewport rows fall entirely past the end of `newLines`
-			// and render as all blanks, hiding the prompt until the next checkpoint. For
-			// shrinks that large, yanking a scrolled reader (historyRebuild) is the lesser
-			// evil; do it unconditionally.
-			if (newLines.length <= this.#scrollbackHighWater) {
+			// That deferral only carries real content when `newLines.length` reaches the
+			// padded viewport top (`previousLines.length - height`) — otherwise every
+			// row the padded repaint draws is past the end of `newLines` and renders as
+			// blank, hiding the prompt until the next checkpoint. This can happen even
+			// when `scrollbackHighWater` is much lower than `previousLines.length - height`,
+			// because prior unknown-POSIX viewport repaints commit longer logical frames
+			// without moving the native scrollback boundary. For shrinks that large,
+			// yanking a scrolled reader (historyRebuild) is the lesser evil; do it
+			// unconditionally.
+			const paddedViewportTop = Math.max(0, this.#previousLines.length - height);
+			if (newLines.length <= paddedViewportTop) {
 				return { kind: "historyRebuild" };
 			}
 			this.#markNativeScrollbackDirty();

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

@@ -1625,6 +1625,61 @@ describe("TUI terminal-state regressions", () => {
 				tui.stop();
 			}
 		});
+		it("rebuilds history when prior POSIX repaint left the padded viewport past the new tail", async () => {
+			const term = new UnknownViewportTerminal(40, 10);
+			const tui = new TUI(term);
+			const initial = rows("line-", 19);
+			const component = new MutableLinesComponent([...initial, "prompt-row"]);
+			tui.addChild(component);
+
+			try {
+				tui.start();
+				await settle(term);
+
+				// Unknown-POSIX offscreen mutation: repainting the viewport commits the
+				// 120-row logical frame, but `#emitViewportRepaint` intentionally does not
+				// advance `#scrollbackHighWater` (it remains at the original 20-row frame's
+				// 10-row overflow). The later shrink must compare against the padded viewport
+				// top (`120 - height`) rather than the stale high-water mark.
+				const expanded = ["edited-line", ...rows("line-", 118), "prompt-row"];
+				component.setLines(expanded);
+				tui.requestRender();
+				await settle(term);
+				expect(visible(term).map(line => line.trim())).toEqual([
+					"line-109",
+					"line-110",
+					"line-111",
+					"line-112",
+					"line-113",
+					"line-114",
+					"line-115",
+					"line-116",
+					"line-117",
+					"prompt-row",
+				]);
+
+				const short = [...rows("short-", 14), "prompt-row"];
+				component.setLines(short);
+				tui.requestRender();
+				await settle(term);
+
+				expect(visible(term).map(line => line.trim())).toEqual([
+					"short-5",
+					"short-6",
+					"short-7",
+					"short-8",
+					"short-9",
+					"short-10",
+					"short-11",
+					"short-12",
+					"short-13",
+					"prompt-row",
+				]);
+				expect(term.getScrollBuffer().join("\n")).not.toContain("line-");
+			} finally {
+				tui.stop();
+			}
+		});
 
 		it("renders streaming row inserts on WSL Windows Terminal even when viewport probe is unavailable", async () => {
 			const originalPlatform = process.platform;