[BUG] Fullscreen render mode: no scrollbar and scrolling fully broken (arrows/PageUp/PageDown do nothing) once output exceeds one screen

What happened

In the fullscreen rendering mode (enabled via the in-app Claude Code option), once the terminal output grows past one screen there is no scrollbar rendered, and scrolling does not work at all. Arrow keys and Page Up / Page Down do nothing, so earlier output becomes inaccessible.

Expected behavior

When content exceeds the viewport, a scrollbar should appear and keyboard scrolling (Arrow keys, Page Up / Page Down) should move the viewport to reveal earlier output.

Steps to reproduce

1. Enable fullscreen rendering mode via the in-app Claude Code option (fullscreen toggle).
2. Run a session that produces enough output to scroll past the visible screen.
3. Observe: no scrollbar appears.
4. Try to scroll up with Arrow keys / Page Up / Page Down — nothing happens.

Environment

• Claude Code version: 2.1.195
• OS: Windows 11 Pro (10.0.26100)
• Terminal: PowerShell 7.6.3 (pwsh)
• Render mode: Fullscreen

Notes

This looks distinct from the existing fullscreen scroll issues, where scrolling still works but is mouse-only or janky:

• #70724 — mouse wheel navigates prompt history with CLAUDE_CODE_DISABLE_MOUSE=1
• #65833 — scroll wheel sends arrow keys instead of scrolling
• #56546 — janky / non-smooth scrolling
• #53699 — laggy scrolling in VSCode integrated terminal

Here keyboard scrolling (arrows / PageUp / PageDown) is entirely non-functional and no scrollbar is rendered at all — not merely janky or mouse-only