fix(tiles): page a hollow tile only from the live screen, review follow-up

A tile counts as hollow when every row above its screen is its own overflow
(baseY minus _overflowRows is 0), so unlike the primary pane, whose hollow
buffer has baseY 0, its viewport can sit above the bottom while it is hollow:
Shift+PageUp, a scrollbar drag or a wheel during the first replay leave it up
there. _maybePageCliTranscript never looked at the viewport, so every wheel,
wheel-down included, was turned into PageUp/PageDown and swallowed. xterm never
scrolled back, the stale rows stayed on screen while the CLI paged out of
view, and clicks were dropped too, because the click report refuses an
off-bottom viewport.

The tile now pages only while _terminalViewportAtBottom holds for its own
terminal, checked before the pending travel is touched. Off the bottom the
wheel stays with xterm, so a wheel-down brings the viewport home and paging
resumes from there. The primary pane is unchanged: its hollow test already
implies a viewport at the bottom, which the twin comment now says.

Tests: a unit case for a tile hollow by the discount with its viewport above
the bottom (no page key, no preventDefault, and no travel carried over once
back home), and the real-browser case now scrolls a hollow tile up and proves
a real wheel-down scrolls xterm home with no page key sent, then pages again.
Both go red with the gate removed, and the unit case also with the gate moved
below the pending-travel update.

Co-Authored-By: Claude Opus 5.5 (1M context) <noreply@anthropic.com>

docs/architecture-invariants.md

@@ -225,7 +225,7 @@ Further detail: the `<prefix>: <title>` form (`w3-myapp: fix the login redirect`
 
 **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 (claude ≥ 2.1.187 while `cliMouseTracking` is true, i.e. fullscreen; 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 hollow pane 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, the `cliMouseTracking` flag is unset (inline claude, or fullscreen right after a server restart), or the user turned on `terminalWheelLocalScrollback`. **opencode is the fifth case, and the gate is false there by design**: its TUI runs on the ALTERNATE SCREEN (1.18.31 measured: tmux `alternate_on=1`, `history_size=0`), so the buffer is hollow, and it IGNORES SGR wheel reports entirely (six `\x1b[<64;…M` reports against an idle pane left the capture byte-identical) while still paging its transcript on PageUp/PageDown (`messages_page_up/down`) — so paging is the only gesture that can reach it, and without it the wheel was silently dead in every opencode tab. The 1.12.0 retest reported exactly that shape for Claude: 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 guard (`_localScrollbackIsHollow()` — `claude` or `opencode`, 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). ⚠️ `shell`/`pi` own real terminal scrollback and are never paged, and codex/gemini/antigravity/grok/deepseek/omp page-key behaviour is unverified (`docs/scrollback-fix-plan.md`). ⚠️ 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()`. **Tiles** (a grid tile, the split's Pane B: `TerminalTile`) page the wheel through these same gates, called with the tile's own terminal and session (`_localScrollbackIsHollow(target)`, `_shouldForwardWheelToApp(ev, target)`) and the pure `CodemanTerminalInput.pageKeysForTravel`, so the mode list stays in terminal-ui.js alone. ⚠️ A tile's `baseY` is rarely 0 even when hollow: its first capture is taken at the PTY's previous, taller size and its row-shrinking fits push rows up, so it passes `localRows` with those rows discounted (`TerminalTile._localRows`). Tiles have no touch path and do not forward SGR wheel (tile-grid-plan follow-up 4). Tests: `test/terminal-scroll-routing.test.ts`, `test/terminal-tile-scroll.test.ts`, `test/claude-cli-version-cache.test.ts`.
+**A false gate on a hollow pane 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, the `cliMouseTracking` flag is unset (inline claude, or fullscreen right after a server restart), or the user turned on `terminalWheelLocalScrollback`. **opencode is the fifth case, and the gate is false there by design**: its TUI runs on the ALTERNATE SCREEN (1.18.31 measured: tmux `alternate_on=1`, `history_size=0`), so the buffer is hollow, and it IGNORES SGR wheel reports entirely (six `\x1b[<64;…M` reports against an idle pane left the capture byte-identical) while still paging its transcript on PageUp/PageDown (`messages_page_up/down`) — so paging is the only gesture that can reach it, and without it the wheel was silently dead in every opencode tab. The 1.12.0 retest reported exactly that shape for Claude: 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 guard (`_localScrollbackIsHollow()` — `claude` or `opencode`, 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). ⚠️ `shell`/`pi` own real terminal scrollback and are never paged, and codex/gemini/antigravity/grok/deepseek/omp page-key behaviour is unverified (`docs/scrollback-fix-plan.md`). ⚠️ 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()`. **Tiles** (a grid tile, the split's Pane B: `TerminalTile`) page the wheel through these same gates, called with the tile's own terminal and session (`_localScrollbackIsHollow(target)`, `_shouldForwardWheelToApp(ev, target)`) and the pure `CodemanTerminalInput.pageKeysForTravel`, so the mode list stays in terminal-ui.js alone. ⚠️ A tile's `baseY` is rarely 0 even when hollow: its first capture is taken at the PTY's previous, taller size and its row-shrinking fits push rows up, so it passes `localRows` with those rows discounted (`TerminalTile._localRows`). ⚠️ So a tile can be hollow with its viewport still up in those rows (Shift+PageUp, a scrollbar drag, a wheel during the first replay), and it pages only while `_terminalViewportAtBottom(tile.terminal)` holds: otherwise every wheel, wheel-down included, was paged and swallowed, the stale rows stayed on screen and the click report (which refuses an off-bottom viewport) went dead too. Off the bottom the wheel stays xterm's, so a wheel-down brings the viewport home. The primary pane needs no such gate: hollow there means `baseY === 0`, which is always at the bottom. Tiles have no touch path and do not forward SGR wheel (tile-grid-plan follow-up 4). Tests: `test/terminal-scroll-routing.test.ts`, `test/terminal-tile-scroll.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.