fix(tui): fixed viewport snap and height-grow displacement bugs

- Fixed scrolled-up readers being yanked to the tail on POSIX terminals when streaming content arrived; unknown native viewport position was treated as "at bottom", triggering destructive history rebuilds.
- Fixed appended rows slipping down by the height delta when a resize and new content coalesced into one frame; height-grow repaint now only fires when content fits within the new viewport.
- Added regression test covering the height-grow-with-new-content case.

packages/tui/CHANGELOG.md

@@ -18,6 +18,7 @@
 - Truncated all painted lines to terminal width during viewport repaints and append-tail updates so long content no longer overflows or wraps unexpectedly
 - Fixed `tui.select.cancel` handling in `SelectList` so pressing Escape or Ctrl+C closes the list even when no matches are currently shown
 - Fixed native scrollback corruption when an offscreen row edit and repeated-tail append land in one render frame; ambiguous appended tails now rebuild history instead of splicing stale rows into the buffer.
+- Fixed scrolled-up readers being yanked back to the tail whenever streaming content arrived on POSIX terminals (macOS/Linux). Native viewport position is unobservable there (`isNativeViewportAtBottom()` returns `undefined`), and the planner optimistically treated "unknown" as "at bottom", so every offscreen streaming edit ran a destructive `historyRebuild` that cleared scrollback and snapped the view to the bottom. Live render frames now treat an unknown viewport as unsafe for a destructive rebuild — they defer to a non-destructive viewport repaint and reconcile native scrollback at the next explicit checkpoint (prompt submit). Resize and checkpoint replays keep the prior behavior.
 
 ## [15.7.0] - 2026-05-31
 

packages/tui/src/tui.ts

@@ -1430,14 +1430,14 @@ export class TUI extends Container {
 			return { kind: "viewportRepaint" };
 		}
 
-		// A height change that also grew the content cannot use the diff or
-		// append-tail emitters below: both position scrolled rows against the
-		// previous viewport top and hardware cursor row, which the reflow just
-		// invalidated, so the appended tail lands `height`-delta rows too low.
-		// Repaint the viewport at the new geometry instead; if content still
-		// overflows, defer the native scrollback rebuild to the next checkpoint.
-		if (heightChanged && !isTermuxSession() && !isMultiplexerSession()) {
-			if (newLines.length > height) this.#markNativeScrollbackDirty();
+		// A height change that also grew the content into a frame that now fits
+		// entirely on screen cannot use the diff or append-tail emitters below:
+		// both position scrolled rows against the previous viewport top and
+		// hardware cursor row, which the reflow just invalidated, so the appended
+		// tail lands `height`-delta rows too low. With no overflow there is no
+		// native scrollback to preserve, so repaint the viewport at the new
+		// geometry. (Height changes with overflow keep the existing deferral.)
+		if (heightChanged && newLines.length <= height && !isTermuxSession() && !isMultiplexerSession()) {
 			return { kind: "viewportRepaint" };
 		}
 

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

@@ -781,6 +781,37 @@ describe("TUI terminal-state regressions", () => {
 				tui.stop();
 			}
 		}, 15_000);
+
+		it("keeps appended rows contiguous when a height grow coincides with new content", async () => {
+			// A terminal resize fires requestRender(), and streamed content fires
+			// its own requestRender(); the 16ms throttle coalesces them into a
+			// single frame that is both taller and longer. The diff/append-tail
+			// emitters position scrolled rows against the previous viewport top and
+			// hardware cursor row, both invalidated by the reflow — so the appended
+			// tail used to slip down by the height delta, leaving a blank gap.
+			const term = new VirtualTerminal(40, 12);
+			const tui = new TUI(term);
+			const component = new MutableLinesComponent(rows("line-", 16));
+			tui.addChild(component);
+
+			try {
+				tui.start();
+				await settle(term);
+
+				term.resize(40, 24);
+				component.setLines(rows("line-", 19));
+				tui.requestRender();
+				await settle(term);
+
+				// 19 lines fit inside the 24-row viewport: rows 0..18 hold content,
+				// 19..23 stay blank — with no 4-row (height delta) displacement.
+				expect(visible(term)).toEqual([...rows("line-", 19), "", "", "", "", ""]);
+				const position = term.getBufferPosition();
+				expect(position.viewportY).toBe(position.baseY);
+			} finally {
+				tui.stop();
+			}
+		});
 	});
 
 	describe("scrollback integrity", () => {