fix(tui): corrected TUI scrollback preservation during rerenders

- Preserved preexisting terminal scrollback during forced and structural rerenders in TUI.#prepareForcedRender.
- Gated historyRebuild triggers on #scrollbackHighWater in TUI.#canReplayNativeScrollbackAtCheckpoint.
- Expanded render-stress coverage with child mutations, viewport variants, and replay-mode scenario parsing.

packages/tui/CHANGELOG.md

@@ -1,7 +1,6 @@
 # Changelog
 
 ## [Unreleased]
-
 ### Added
 
 - Added `overflowSearch` to `SelectListLayoutOptions` to let consumers enable or disable type-to-filter search and search-status rendering per SelectList instance
@@ -13,6 +12,7 @@
 
 ### Fixed
 
+- Preserved existing terminal scrollback during forced and structural TUI renders so preexisting shell lines remained visible after component mutations
 - Rebuilt native scrollback for safe bottom-anchored offscreen edits and high-water preview collapses instead of repainting only the viewport, preventing stale or duplicated rows above the live viewport.
 - Stripped internal cursor marker sentinels from all rendered lines so offscreen focus markers no longer leak into terminal output
 - Truncated all painted lines to terminal width during viewport repaints and append-tail updates so long content no longer overflows or wraps unexpectedly

packages/tui/src/tui.ts

@@ -705,8 +705,9 @@ export class TUI extends Container {
 
 	#prepareForcedRender(clearScrollback: boolean): void {
 		this.#clearScrollbackOnNextRender ||= clearScrollback;
+		const droppedLines = this.#previousLines.length > 0;
 		this.#previousLines = [];
-		this.#previousLinesDroppedForForcedRender = true;
+		this.#previousLinesDroppedForForcedRender = droppedLines;
 		this.#previousWidth = -1; // -1 triggers widthChanged, forcing a full clear
 		this.#previousHeight = -1; // -1 triggers heightChanged, forcing a full clear
 		this.#cursorRow = 0;
@@ -1382,6 +1383,7 @@ export class TUI extends Container {
 			}
 			if (
 				newLines.length !== this.#previousLines.length &&
+				this.#scrollbackHighWater > 0 &&
 				this.#canReplayNativeScrollbackAtCheckpoint(nativeViewportAtBottom, allowUnknownViewportMutation)
 			) {
 				return { kind: "historyRebuild" };