Window.vertical_scroll is read-only in prompt_toolkit.
Use custom scroll_offset to track position and slice output lines.
- scroll_offset=0 shows newest content (auto-scroll)
- Page Up increases offset (scroll into history)
- Page Down decreases offset (towards newest)
- Home/End jump to top/bottom
Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>