large behavior change to how the TUI owns its viewport, history, and suspend behavior.
Core model is in place; a few items are still being polished before this is ready to merge.

We've moved this over to a new tui2 crate from being directly on the tui crate.
To enable use --enable tui2 (or the equivalent in your config.toml). See https://developers.openai.com/codex/local-config#feature-flags

Note that this serves as a baseline for the changes that we're making to be applied rapidly. Tui2 may not track later changes in the main tui. It's experimental and may not be where we land on things.

Summary

This PR moves the Codex TUI off of “cooperating” with the terminal’s scrollback and onto a model
where the in‑memory transcript is the single source of truth. The TUI now owns scrolling, selection,
copy, and suspend/exit printing based on that transcript, and only writes to terminal scrollback in
append‑only fashion on suspend/exit. It also fixes streaming wrapping so streamed responses reflow
with the viewport, and introduces configuration to control whether we print history on suspend or
only on exit.

High‑level goals:

• Ensure history is complete, ordered, and never silently dropped.
• Print each logical history cell at most once into scrollback, even with resizes and suspends.
• Make scrolling, selection, and copy match the visible transcript, not the terminal’s notion of
  scrollback.
• Keep suspend/alt‑screen behavior predictable across terminals.

Core Design Changes

Transcript & viewport ownership

• Treat the transcript as a list of cells (user prompts, agent messages, system/info rows,
  streaming segments).
• On each frame:
  • Compute a transcript region as “full terminal frame minus the bottom input area”.
  • Flatten all cells into visual lines plus metadata (which cell + which line within that cell).
  • Use scroll state to choose which visual line is at the top of the region.
  • Clear that region and draw just the visible slice of lines.
• The terminal’s scrollback is no longer part of the live layout algorithm; it is only ever written
  to when we decide to print history.

User message styling

• User prompts now render as clear blocks with:
  • A blank padding line above and below.
  • A full‑width background for every line in the block (including the prompt line itself).
• The same block styling is used when we print history into scrollback, so the transcript looks
  consistent whether you are in the TUI or scrolling back after exit/suspend.

Scrolling, Mouse, Selection, and Copy

Scrolling

• Scrolling is defined in terms of the flattened transcript lines:
  • Mouse wheel scrolls up/down by fixed line increments.
  • PgUp/PgDn/Home/End operate on the same scroll model.
• The footer shows:
  • Whether you are “following live output” vs “scrolled up”.
  • Current scroll position (line / total).
• When there is no history yet, the bottom pane is pegged high and gradually moves down as the
  transcript fills, matching the existing UX.

Selection

• Click‑and‑drag defines a linear selection over transcript line/column coordinates, not raw
  screen rows.
• Selection is content‑anchored:
  • When you scroll, the selection moves with the underlying lines instead of sticking to a fixed
    Y position.
  • This holds both when scrolling manually and when new content streams in, as long as you are in
    “follow” mode.
• The selection only covers the “transcript text” area:
  • Left gutter/prefix (bullets, markers) is intentionally excluded.
  • This keeps copy/paste cleaner and avoids including structural margin characters.

Copy (Ctrl+Y)

• Introduce a small clipboard abstraction (ClipboardManager‑style) and use a cross‑platform
  clipboard crate under the hood.
• When Ctrl+Y is pressed and a non‑empty selection exists:
  • Re‑render the transcript region off‑screen using the same wrapping as the visible viewport.
  • Walk the selected line/column range over that buffer to reconstruct the exact text:
    • Includes spaces between words.
    • Preserves empty lines within the selection.
  • Send the resulting text to the system clipboard.
  • Show a short status message in the footer indicating success/failure.
• Copy is best‑effort:
  • Clipboard failures (headless environment, sandbox, remote sessions) are handled gracefully via
    status messages; they do not crash the TUI.
  • Copy does not insert a new history entry; it only affects the status bar.

Streaming and Wrapping

Previous behavior

Previously, streamed markdown:

• Was wrapped at a fixed width at commit time inside the streaming collector.
• Those wrapped Line<'static> values were then wrapped again at display time.
• As a result, streamed paragraphs could not “un‑wrap” when the terminal width increased; they were
  permanently split according to the width at the start of the stream.

New behavior

This PR implements the first step from codex-rs/tui/streaming_wrapping_design.md:

• Streaming collector is constructed without a fixed width for wrapping.
  • It still:
    • Buffers the full markdown source for the current stream.
    • Commits only at newline boundaries.
    • Emits logical lines as new content becomes available.
• Agent message cells now wrap streamed content only at display time, based on the current
  viewport width, just like non‑streaming messages.
• Consequences:
  • Streamed responses reflow correctly when the terminal is resized.
  • Animation steps are per logical line instead of per “pre‑wrapped” visual line; this makes some
    commits slightly larger but keeps the behavior simple and predictable.

Streaming responses are still represented as a sequence of logical history entries (first line +
continuations) and integrate with the same scrolling, selection, and printing model.

Printing History on Suspend and Exit

High‑water mark and append‑only scrollback

• Introduce a cell‑based high‑water mark (printed_history_cells) on the transcript:
  • Represents “how many cells at the front of the transcript have already been printed”.
  • Completely independent of wrapped line counts or terminal geometry.
• Whenever we print history (suspend or exit):
  • Take the suffix of transcript_cells beyond printed_history_cells.
  • Render just that suffix into styled lines at the current width.
  • Write those lines to stdout.
  • Advance printed_history_cells to cover all cells we just printed.
• Older cells are never re‑rendered for scrollback. They stay in whatever wrapping they had when
  printed, which is acceptable as long as the logical content is present once.

Suspend (Ctrl+Z)

• On suspend:
  • Leave alt screen if active and restore normal terminal modes.
  • Render the not‑yet‑printed suffix of the transcript and append it to normal scrollback.
  • Advance the high‑water mark.
  • Suspend the process.
• On resume (fg):
  • Re‑enter the TUI mode (alt screen + input modes).
  • Clear the viewport region and fully redraw from in‑memory transcript and state.

This gives predictable behavior across terminals without trying to maintain scrollback live.

Exit

• On exit:
  • Render any remaining unprinted cells once and write them to stdout.
  • Add an extra blank line after the final Codex history cell before printing token usage, so the
    transcript and usage info are visually separated.
• If you never suspended, exit prints the entire transcript exactly once.
• If you suspended one or more times, exit prints only the cells appended after the last suspend.

Configuration: Suspend Printing

This PR also adds configuration to control when we print history:

• New TUI config option to gate printing on suspend:
  • At minimum:
    • print_on_suspend = true – current behavior: print new history at each suspend and on exit.
    • print_on_suspend = false – only print on exit.
  • Default is tuned to preserve current behavior, but this can be revisited based on feedback.
• The config is respected in the suspend path:
  • If disabled, suspend only restores terminal modes and stops rendering but does not print new
    history.
  • Exit still prints the full not‑yet‑printed suffix once.

This keeps the core viewport logic agnostic to preference, while letting users who care about
quiet scrollback opt out of suspend printing.

Tradeoffs

What we gain:

• A single authoritative history model (the in‑memory transcript).
• Deterministic viewport rendering independent of terminal quirks.
• Suspend/exit flows that:
  • Print each logical history cell exactly once.
  • Work across resizes and different terminals.
  • Interact cleanly with alt screen and raw‑mode toggling.
• Consistent, content‑anchored scrolling, selection, and copy.
• Streaming messages that reflow correctly with the viewport width.

What we accept:

• Scrollback may contain older cells wrapped differently than newer ones.
• Streaming responses appear in scrollback as a sequence of blocks corresponding to their streaming
  structure, not as a single retroactively reflowed paragraph.
• We do not attempt to rewrite or reflow already‑printed scrollback.

For deeper rationale and diagrams, see docs/tui_viewport_and_history.md and
codex-rs/tui/streaming_wrapping_design.md.

Still to Do Before This PR Is Ready

These are scoped to this PR (not long‑term future work):

• Streaming wrapping polish

  • Double‑check all streaming paths use display‑time wrapping only.
  • Ensure tests cover resizing after streaming has started.

• Suspend printing config

  • Finalize config shape and default (keep existing behavior vs opt‑out).
  • Wire config through TUI startup and document it in the appropriate config docs.

• Bottom pane positioning

  • Ensure the bottom pane is pegged high when there’s no history and smoothly moves down as the
    transcript fills, matching the current behavior across startup and resume.

• Transcript mouse scrolling

  • Re‑enable wheel‑based transcript scrolling on top of the new scroll model.
  • Make sure mouse scroll does not get confused with “alternate scroll” modes from terminals.

• Mouse selection vs streaming

  • When selection is active, stop auto‑scrolling on streaming so the selection remains stable on
    the selected content.
  • Ensure that when streaming continues after selection is cleared, “follow latest output” mode
    resumes correctly.

• Auto‑scroll during drag

  • While the user is dragging a selection, auto‑scroll when the cursor is at/near the top or bottom
    of the transcript viewport to allow selecting beyond the current visible window.

• Feature flag / rollout

  • Investigate gating the new viewport/history behavior behind a feature flag for initial rollout,
    so we can fall back to the old behavior if needed during early testing.

• Before/after videos

  • Capture short clips showing:
    • Scrolling (mouse + keys).
    • Selection and copy.
    • Streaming behavior under resize.
    • Suspend/resume and exit printing.
  • Use these to validate UX and share context in the PR discussion.