[Architecture] oclite: Scroll region layout with fixed bottom panel

[randomm]

Description

Implement ANSI scroll region architecture for oclite — scrollable conversation area with fixed bottom panel.

Problem

After removing alt screen buffer (#66), oclite writes everything append-only to the main terminal buffer. This means:

• ❌ Cannot rewrite tool status lines once scrolled
• ❌ Cannot have a persistent status bar
• ❌ Cannot show live metrics during streaming
• ❌ Cannot have a fixed input area

Solution: ANSI Scroll Regions

Use \x1b[{top};{bottom}r to define a scroll region that covers most of the terminal, with 2-3 fixed lines at the bottom.

Layout:

┌─────────────────────────────────────┐
│ Scrollable conversation area        │ ← Scroll region (rows 1 to N-3)
│ User messages, agent responses      │
│ Tool output, markdown text          │
│ ...scrolls naturally...             │
│                                     │
├─────────────────────────────────────┤
│ ◆ Thinking · 5s · 3/5 tools        │ ← Fixed status line (row N-2)
│ project-manager ❯ █                │ ← Fixed input line (row N-1)
│ Esc cancel · Shift+Tab agents      │ ← Fixed hint bar (row N)
└─────────────────────────────────────┘

Requirements

Scroll Region Setup

• On startup: query terminal size, set scroll region \x1b[1;{rows-3}r
• Handle SIGWINCH (terminal resize) — recalculate and reset scroll region
• On cleanup: reset scroll region to full terminal \x1b[r

Fixed Bottom Panel (3 lines)

• Status line: During streaming shows spinner + elapsed time + tool count + token count. During idle shows nothing or session info.
• Input line: Prompt + user input (project-manager ❯ █)
• Hint bar: Context-sensitive keyboard shortcuts (Esc cancel · Shift+Tab agents · /help)

Writing to Scroll Region

• All conversation output (text, tool display, errors) writes to scroll region
• Scroll region scrolls naturally — old content preserved in scroll buffer
• New content appends at bottom of scroll region, pushing old content up

Writing to Fixed Panel

• Use \x1b[s (save cursor) before updating fixed lines
• Move to fixed line with \x1b[{row};1H (absolute position)
• Write content + clear to end of line
• Use \x1b[u (restore cursor) to return to scroll region
• Update fixed panel without disturbing scroll region content

Terminal Resize Handling

• Listen for SIGWINCH signal
• Recalculate scroll region boundaries
• Redraw fixed bottom panel
• Preserve scroll position in conversation area

Implementation Plan

1. Create src/cli/lite/layout.ts — scroll region manager
2. Modify index.ts — use layout manager for all writes
3. Modify spinner.ts — render in status line instead of scroll region
4. Modify bottombar.ts — render in fixed input/hint lines

Depends On

• None (foundational)

Blocks

• [UX] oclite: Rich status bar with streaming metrics #67 Rich status bar (renders in fixed status line)
• [UX] oclite: Live subagent status panel #68 Subagent panel (renders in scroll region or status line)
• [UX] oclite: Bottom context bar with workspace info #69 Bottom context bar (renders in fixed hint bar)

Technical Reference

• ANSI scroll region: \x1b[{top};{bottom}r (DEC origin mode)
• Terminal size: process.stdout.rows, process.stdout.columns
• Resize signal: process.on('SIGWINCH', handler)
• Save/restore cursor: \x1b[s / \x1b[u
• Absolute position: \x1b[{row};{col}H

Files

• New: packages/opencode/src/cli/lite/layout.ts
• Modified: packages/opencode/src/cli/lite/index.ts
• Modified: packages/opencode/src/cli/lite/spinner.ts
• Modified: packages/opencode/src/cli/lite/bottombar.ts

[randomm]

Superseded by #73 (revert) and #74 (adopt append-only model). The scroll region approach is architecturally wrong for this use case — see #76 for research evidence.