Auto-snap child viewport to bottom when typing into scrollback

Typing into a focused child while its emulator viewport was
scrolled up left the keystroke heading to the PTY but the input
box invisible below the visible region — it looked like typing
did nothing. processStdin's flushForward now sets
pendingViewportBottom whenever bytes are actually injected, so
the existing post-loop handler snaps the viewport and repaints.

Wheel events and Ctrl-B paths are untouched: both are intercepted
before reaching forward, so wheel still scrolls into history and
Ctrl-B is still the explicit escape hatch. Only bytes that would
actually reach the child PTY trigger the snap.

CHANGELOG.md

@@ -7,6 +7,12 @@ loosely follows [Semantic Versioning](https://semver.org/spec/v2.0.0.html).
 ## [Unreleased]
 
 ### Fixed
+- Typing into a focused child while its emulator viewport is
+  scrolled up into scrollback history now auto-snaps the viewport
+  back to the live area. Previously the keystroke reached the
+  child PTY but the input box was off-screen below the visible
+  region, so it looked like typing did nothing. Wheel scrolling
+  and Ctrl-B are unchanged; only forwarded keystrokes snap.
 - Top tab bar now keeps the top-level agent's tab highlighted
   when focus is on one of its sub-agents (or on a Processes pane
   entry, matching the existing agent-tree behavior). Previously

internal/app/app.go

@@ -1323,6 +1323,15 @@ func (st *uiState) processStdin(chunk []byte) {
 	}
 
 	forward := make([]byte, 0, len(chunk))
+
+	var pendingAction *paletteAction
+	var pendingNav navEntry
+	var pendingRestartID string
+	var pendingViewportDelta int
+	var pendingViewportBottom bool
+	var pendingPadStep int
+	var pendingPadExit bool
+
 	flushForward := func() {
 		if len(forward) == 0 {
 			return
@@ -1337,19 +1346,16 @@ func (st *uiState) processStdin(chunk []byte) {
 				if prev != OwnerUser {
 					go st.drawStatusLine()
 				}
+				// Auto-snap the emulator viewport to the live area
+				// on any forwarded keystroke. Without this, typing
+				// while scrolled into history leaves the cursor /
+				// echoed bytes off-screen below the visible region.
+				pendingViewportBottom = true
 			}
 		}
 		forward = forward[:0]
 	}
 
-	var pendingAction *paletteAction
-	var pendingNav navEntry
-	var pendingRestartID string
-	var pendingViewportDelta int
-	var pendingViewportBottom bool
-	var pendingPadStep int
-	var pendingPadExit bool
-
 	// childOnPrimary captures whether the focused child is on its primary
 	// screen at the start of this chunk. Wheel events on the primary
 	// screen scroll the emulator viewport (inline scrollback); on the