fix(terminal): stop the scroll-to-top re-pull from deleting history, page the CLI when local scrollback is hollow (#205)

The 1.12.0 retest on #205 reported it still broken in two shapes: a wheel that
did nothing at all on Firefox/macOS (while Fn+Up paged back through intact
text), and iPhone history that went back a little, repeated blocks and got
worse the further up it went. Both come from a Claude pane's LOCAL buffer being
hollow: tmux keeps no history for a repaint-mode pane (history_size 0), so
xterm holds only replayed repaint frames.

1. The scroll-to-top full=1 re-pull now refuses a DOWNGRADE. It resets the
   terminal and rewrites it from the capture, which is a win when tmux holds
   more than the browser, but for a repaint-mode pane that capture is roughly
   ONE frame and the rewrite deleted history mid-scroll. Measured A/B on a live
   pane, same gesture: guard off collapses 341 rows to 42, guard on preserves
   all 341. _replayWouldShrinkBuffer() estimates the capture's rendered rows
   (escapes stripped, capture-pane -J re-wrapping accounted for) and skips the
   rewrite when it is more than one screen short; a refused session's cooldown
   goes from 4s to 60s so a hollow pane stops re-fetching megabytes.

2. A false forwarding gate on a Claude session no longer means a dead gesture.
   Under a triple guard (claude mode, gate false, baseY 0), wheel and touch
   travel becomes coalesced PageUp/PageDown through the same 40ms queue as the
   SGR reports, at half a screen of travel per page key. Shift is excluded: it
   keeps meaning "local scrollback".

3. getClaudeCliVersion() no longer caches FAILURE. It stored null on any
   exception and guarded on !== undefined, so one timed-out or PATH-starved
   probe at the first Claude session start disabled wheel-forwarding for every
   Claude session until the server restarted, which fits a report of breakage on
   phone, tablet and laptop at once. Success is still cached for the process
   lifetime; failures retry with a 1/2/4 up to 15min backoff, and the policy is
   a pure function so the semantics are testable without spawning claude.

4. The terminalWheelLocalScrollback footgun is handled by pairing rather than
   scoping: the setting keeps meaning exactly what it says, and fix 2 catches
   the case where "local" is empty. The App Settings tooltip now says to leave
   it off for Claude/Codex sessions.

5. _logScrollRouting() prints one line per session per distinct decision:
   forward-sgr / page-keys / local-scrollback / repull-refused-downgrade, with
   mode, cliVersion, the opt-out state, mouse tracking and local scrollback
   depth. #205 ran two rounds of remote guesswork over questions that line
   answers directly.

Verified end to end against a real isolated instance (own data dir and tmux
socket) with real wheel events: forwarding still sends SGR reports, the opt-out
now sends real PageUp/PageDown where the wheel was dead, a tab-switch collapse
(401 rows to 44) is still fully recovered by the re-pull (back to 401), and a
seeded 341-row Claude buffer survives the same gesture that destroys it with the
guard disabled.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

docs/architecture-invariants.md

@@ -58,7 +58,7 @@ Implementation detail extracted from `CLAUDE.md` so that file stays small enough
 
 ### Full-scrollback replay
 
-**Full-scrollback replay** (COD-164/#148, reworked for #205): `GET /api/sessions/:id/terminal?full=1` returns the ENTIRE tmux scrollback (capture-pane `-e -S -<lines>` bounded by the configured history limit, explicit `maxBuffer` from the terminal-history config, early byte-cap before normalization, CRLF-normalized for shell panes). On success the capture is returned ALONE (`source='mux-full-history'` — it supersedes the byte buffer; no duplication). The first load OF EACH SESSION per page load requests `full=1` (`_fullHistoryLoaded` Set in app.js — the old one-shot `_initialFullBufferLoad` flag was consumed by whichever tab auto-selected, leaving every other tab one frame of history); later switches keep the cheap `?tail=` visible-frame path. On top of that, scrolling up while already at the TOP of the buffer re-pulls `full=1` on demand (`_maybeRefetchFullHistory`, 4s per-session cooldown, in-flight + tab-switch guards, viewport position held across the replay). The re-pull exists because xterm's buffer is only a WINDOW onto tmux's history and two things shrink it: tmux coalesces bursty output into pane REPAINTS that overwrite rows instead of emitting linefeeds (measured: a 60-line burst added 1 row of browser scrollback and destroyed 34), and a tab switch replays only the visible frame. tmux's own history is intact throughout — the browser just has to ask for it again. On-demand rather than automatic because at a 100k history limit the capture can be megabytes. Tests: `test/tmux-capture-full-history.test.ts`, `test/tmux-scrollback-eol.test.ts`.
+**Full-scrollback replay** (COD-164/#148, reworked for #205): `GET /api/sessions/:id/terminal?full=1` returns the ENTIRE tmux scrollback (capture-pane `-e -S -<lines>` bounded by the configured history limit, explicit `maxBuffer` from the terminal-history config, early byte-cap before normalization, CRLF-normalized for shell panes). On success the capture is returned ALONE (`source='mux-full-history'` — it supersedes the byte buffer; no duplication). The first load OF EACH SESSION per page load requests `full=1` (`_fullHistoryLoaded` Set in app.js — the old one-shot `_initialFullBufferLoad` flag was consumed by whichever tab auto-selected, leaving every other tab one frame of history); later switches keep the cheap `?tail=` visible-frame path. On top of that, scrolling up while already at the TOP of the buffer re-pulls `full=1` on demand (`_maybeRefetchFullHistory`, 4s per-session cooldown, in-flight + tab-switch guards, viewport position held across the replay). The re-pull exists because xterm's buffer is only a WINDOW onto tmux's history and two things shrink it: tmux coalesces bursty output into pane REPAINTS that overwrite rows instead of emitting linefeeds (measured: a 60-line burst added 1 row of browser scrollback and destroyed 34), and a tab switch replays only the visible frame. tmux's own history is intact throughout — the browser just has to ask for it again. On-demand rather than automatic because at a 100k history limit the capture can be megabytes. ⚠️ **The re-pull must never DOWNGRADE the buffer** (#205 round 2): the same reasoning that makes it a win for a shell pane makes it destructive for a repaint-mode CLI pane, where tmux keeps no history of its own (`history_size≈0` measured for a Claude pane) and the capture is roughly ONE frame while xterm may hold hundreds of rows of replayed frames — `_resetTerminalForReplay()` + rewrite then deletes history mid-scroll ("goes back a bit, repeats blocks, gets worse the further up I go"; measured A/B on a live pane: 341 rows → 42 with the guard off). `_replayWouldShrinkBuffer()` (terminal-ui.js) estimates the capture's rendered rows — escape sequences stripped, `capture-pane -J` re-wrapping accounted for — and the pull is skipped when that is more than one screen short of `buffer.active.length`. The one-screen tolerance matters: both sides are estimates (the buffer length counts trailing blank rows), so only a clear downgrade is refused. A refused session joins `_fullHistoryRepullUseless`, raising its cooldown from 4s to 60s so a hollow pane stops re-fetching megabytes on every scroll-up. Tests: `test/tmux-capture-full-history.test.ts`, `test/tmux-scrollback-eol.test.ts`, `test/terminal-scroll-routing.test.ts`.
 
 ### Terminal scrollback: strip flavors and wheel/touch forwarding
 
@@ -66,6 +66,10 @@ Implementation detail extracted from `CLAUDE.md` so that file stays small enough
 
 **Wheel/touch forwarding is NOT gated on viewport-at-bottom** (#205, `terminal-ui.js:_shouldForwardWheelToApp`): for sessions verified to scroll their own transcript on SGR wheel reports (codex, claude ≥ 2.1.187 — version via the local/docker/remote `--version` probes), the plain wheel AND touch drags forward as coalesced SGR reports (`_forwardScrollToApp` → `_sendSyntheticSgrWheel`, 40ms batches, 5-tick cap, 512-byte queue bound). It used to gate on the viewport being at the bottom so both scrollbacks stayed reachable, but a repaint-mode CLI keeps NO terminal scrollback of its own — xterm's buffer holds only replayed repaint frames, so local scrolling drags the CLI's pinned prompt box up the screen over stale frames; and `scrollToLastNonEmptyLine()` routinely parked the viewport off-bottom, silently pinning the wheel to local. Forwarding now snaps the viewport home first (SGR coordinates address the LIVE screen — a report computed from a scrolled-up viewport would hit-test the wrong row). Local scrollback remains on Shift+wheel and the `terminalWheelLocalScrollback` opt-out (both also cover touch via the shared gate; touch has no Shift, so the setting is its only local pin). `_wheelScrollLines()` normalizes `deltaMode` (Firefox fires LINE deltas ≈3/notch — read as pixels that rounded to 0 and fell to the ±1 fallback, ~4× too slow; PAGE deltas scale by `terminal.rows`) while keeping the #154 Shift-axis trap (macOS trackpads put Shift+scroll magnitude on deltaX). Tests: `test/terminal-touch-tap.test.ts`.
 
+**A false gate on a Claude session must not mean a DEAD gesture** (#205 round 2, `_maybePageCliTranscript`): every way `_shouldForwardWheelToApp()` returns false leaves a repaint-mode pane scrolling a buffer that has nothing in it (`baseY === 0`) — the version probe came back empty, the CLI really is older than 2.1.187, or the user turned on `terminalWheelLocalScrollback`. The 1.12.0 retest reported exactly that: a wheel that did nothing at all while Fn+Up (PageUp) paged back through intact text, which is the proof that the CLI's own history and the PTY input path were both fine. So under the triple guard (claude mode + gate false + `baseY === 0`) wheel and touch travel is translated into coalesced `\x1b[5~` / `\x1b[6~` through the same 40ms queue as the SGR reports, at half a screen of travel per page key (the key jumps a whole screen; a 1:1 mapping was unusably slow with a discrete wheel). ⚠️ Shift is excluded on purpose — it is the explicit "give me local scrollback" gesture and must keep that meaning. ⚠️ `terminalWheelLocalScrollback` is deliberately NOT scoped away from repaint-mode CLIs even though it is a footgun there: that would silently override an explicit user choice, so the fallback catches it instead. **Server-side counterpart**: `getClaudeCliVersion()` caches SUCCESS for the process lifetime but must never cache FAILURE — it used to, so one timed-out or PATH-starved probe at the first Claude session start disabled wheel-forwarding for every Claude session until the server restarted (a dead wheel on phone, tablet and laptop at once, the signature of a server-side cause). Failures now retry with a 1/2/4…15min backoff; the policy is the pure `resolveClaudeCliVersion()`. Tests: `test/terminal-scroll-routing.test.ts`, `test/claude-cli-version-cache.test.ts`.
+
+**Why the wheel went where it went is LOGGED** (`_logScrollRouting`): one console line per session per distinct decision — `[scroll] <id> → forward-sgr|page-keys|local-scrollback|repull-refused-downgrade (mode=…, cliVersion=…, localScrollbackOptOut=…, mouseTracking=…, localScrollbackRows=…)`. #205 ran two rounds of remote guesswork over questions this line answers directly; keep it when touching the routing.
+
 **The wheel listener is CAPTURE-phase and Codeman owns the scroll** (#205 follow-up, measured on the live instance): xterm's viewport is a vscode-style ScrollableElement that consumes wheel events itself (preventDefault + stopPropagation) whenever it believes a scrollbar exists, ignores `attachCustomWheelEventHandler`, and goes DEAF after `terminal.reset()` — a tab switch or full-history replay leaves its scroll dimensions stale, after which wheel events neither scroll nor propagate reliably. A bubble-phase container listener therefore never fired once local scrollback existed (forwarding, deltaMode and the top-of-buffer re-pull all silently dead exactly on sessions WITH history), and after a tab switch nothing scrolled at all ("works at first, breaks after a tab switch"). The container wheel listener is `{capture: true}`, stops propagation, and scrolls locally via buffer-level `terminal.scrollLines()` (immune to the stale scroller). ⚠️ Two cases are deliberately passed through untouched, in this order BEFORE preventDefault: `mouseTrackingMode !== 'none'` (xterm's encoder forwards the wheel to the PTY — htop/vim with mouse on) and `buffer.active.type === 'alternate'` (direct-PTY vim/less: xterm's alt-scroll converts the wheel to cursor keys). Do not "simplify" this back to a bubble listener or re-delegate local scrolling to xterm's viewport. E2E guard: the reload → tab-switch → wheel matrix in the #205 verification scripts.
 
 ### Run launch synchronization