### Problem
Ctrl+T transcript overlay can omit in-flight coalesced tool calls because it
renders only committed transcript cells while the main viewport can render the
current in-flight ChatWidget.active_cell immediately.
### Mental model
The UI has both committed transcript cells (finalized HistoryCell entries) and
an in-flight active cell that can mutate in place while streaming, often
representing a coalesced exec/tool group. The transcript overlay renders
committed cells plus a render-only live tail derived from the current active
cell. The live tail is cached and only recomputed when its cache key changes,
which is derived from terminal width (wrapping), active-cell revision
(in-place mutations), stream continuation (spacing), and animation tick
(time-based visuals).
### Non-goals
This does not change coalescing rules, flush boundaries, or when active cells
become committed. It does not change tool-call semantics or transcript
persistence; it is a rendering-only improvement for the overlay.
### Tradeoffs
This adds cache invalidation complexity: correctness depends on bumping an
active-cell revision (and/or providing an animation tick) when the active cell
mutates in place. The mechanism is implemented in both codex-tui and codex-tui2,
which keeps behavior consistent but risks drift if future changes are not
applied in lockstep.
### Architecture
App special-cases transcript overlay draws to sync a live tail from ChatWidget
into TranscriptOverlay. TranscriptOverlay remains the owner of committed
transcript cells; the live tail is an optional appended renderable.
HistoryCell::transcript_animation_tick() allows time-dependent transcript output
(spinner/shimmer) to invalidate the cached tail without requiring data mutation.
### Observability
Manual verification is to open Ctrl+T while an exploring/coalesced active cell
is still in-flight and confirm the overlay includes the same in-flight tool-call
group the main viewport shows. The overlay is kept in sync by App passing an
active-cell key and transcript lines into TranscriptOverlay::sync_live_tail; the
key must change when the active cell mutates or animates.
### Tests
Snapshot tests validate that the transcript overlay renders a live tail appended
after committed cells and that identical keys short-circuit recomputation. Unit
tests validate that active-cell revision bumps occur on specific in-place
mutations (e.g. unified exec wait cell command display becoming known late) so
cached tails are invalidated.
## Documentation patches (module, type, function)
### Module-level docs (invariants + mechanisms)
- codex-rs/tui/src/app_backtrack.rs:1
- codex-rs/tui/src/chatwidget.rs:1
- codex-rs/tui/src/pager_overlay.rs:1
- codex-rs/tui/src/history_cell.rs:1
- codex-rs/tui2/src/app_backtrack.rs:1
- codex-rs/tui2/src/chatwidget.rs:1
- codex-rs/tui2/src/pager_overlay.rs:1
- codex-rs/tui2/src/history_cell.rs:1
### Type-level docs (cache key + invariants)
- codex-rs/tui/src/chatwidget.rs (ChatWidget.active_cell_revision, ActiveCellTranscriptKey)
- codex-rs/tui/src/pager_overlay.rs (TranscriptOverlay live tail storage model)
- codex-rs/tui/src/history_cell.rs (HistoryCell::transcript_animation_tick, UnifiedExecWaitCell::update_command_display)
- Mirrored in codex-rs/tui2/src/chatwidget.rs, codex-rs/tui2/src/pager_overlay.rs, codex-rs/tui2/src/history_cell.rs
### Function-level docs (why/when/guarantees/pitfalls)
- codex-rs/tui/src/app_backtrack.rs (overlay_forward_event)
- codex-rs/tui/src/chatwidget.rs (active_cell_transcript_key, active_cell_transcript_lines)
- codex-rs/tui/src/pager_overlay.rs (sync_live_tail, take_live_tail_renderable)
- codex-rs/tui/src/history_cell.rs (transcript_animation_tick, UnifiedExecWaitCell::update_command_display)
- Mirrored in codex-rs/tui2 equivalents where present
### Validation performed
- cd codex-rs && just fmt
- cd codex-rs && cargo test -p codex-tui
- cd codex-rs && cargo test -p codex-tui2
## Design inconsistencies / risks
- Cache invalidation is a distributed responsibility: any future in-place active
cell transcript mutation that forgets to bump active_cell_revision (or expose
an animation tick) can leave the transcript overlay live tail out of sync with
the main viewport.
- TranscriptOverlay tail handling assumes a structural invariant that the live
tail, when present, is exactly one trailing renderable after the committed cell
renderables; if renderable construction changes in a way that violates that
assumption, tail insertion/removal logic becomes incorrect.
- codex-tui and codex-tui2 duplicate the live-tail mechanism; the documentation
is aligned, but the implementation can still drift unless changes continue to
be applied in lockstep.
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