Reconnect the terminal view when somebody comes back to it

Going back to a terminal left alone for a while found it stuck on
"Reconnecting the terminal view…", and stuck is the right word: the banner
stayed and nothing behind it was reconnecting.

The page's whole recovery story was a setTimeout chain, and a chain is exactly
what a WebView is entitled to stop running. Chromium throttles timers in a page
nobody is looking at — down to once a minute once it has been hidden five
minutes — and a renderer that is frozen, or reclaimed and not yet reloaded,
runs none of them. So the socket drops while nobody is watching, the banner
goes up, the retry is scheduled, and the retry is then the one thing not
running.

Three defects, each of which leaves that banner up for the rest of the page's
life.

◆ NOTHING LISTENED FOR THE PAGE COMING BACK. The only thing that could clear
the banner was a timer that may never fire. terminal.js now reconnects on
visibilitychange, focus and online — the events that mean somebody is looking
again, and the ones that cannot be throttled — cancelling the pending timer and
resetting the backoff. Over a healthy socket all three do nothing, which is
what makes them safe to fire as often as clicking a window does.

◆ A HANDSHAKE THAT NEVER FINISHED WAS INVISIBLE. Every retry was scheduled by a
close or an error, so an attempt parked in CONNECTING — which is what a
suspended renderer leaves behind — scheduled nothing at all, ever. There is now
a five-second watchdog on the handshake.

◆ STALE SOCKETS SCHEDULED RETRIES, AND THAT ONE IS A LOOP RATHER THAN A STALL.
connect() never detached the old socket's handlers, and the host aborts the
displaced socket on takeover — TerminalDataPlane.UpgradeAsync, doing exactly
what it should. That close read as a fresh failure and scheduled a retry
against the socket that had just succeeded, whose own close scheduled the next:
no fixed point, reconnecting every second forever with the banner up for most
of it. Every handler now asks whether it is still the page's own attempt, and
connect() closes what it abandons.

◆ WHICH OF THE PLATFORM BEHAVIOURS ACTUALLY BIT IS NOT ESTABLISHED, and the fix
does not depend on knowing. Throttled timers, a frozen renderer and a reclaimed
one all end at the same dead timer; guessing between them would have produced a
narrower fix for one of the three.

THE TEST RUNS terminal.js ITSELF, in a fake browser, inside dotnet test.
RendererPage loads the file the shell project ships — not a transcription of its
logic into C#, which would be a copy that stays correct while the page rots —
into a Jint engine, one per test, over a harness that fakes a WebSocket and a
clock and nothing else. Jint rather than a node script because CI would run the
node one and nobody's inner loop would; the cost is that Jint is not Chromium,
so this proves the page's logic and nothing about how a WebView behaves. That
line is drawn in RendererPage's remark and picked up by two new manual checks,
1.10 for the desktop and 11.12a for the phone, which own the platform half.

Four of the nine tests fail against the page as it stood — the stale close, the
parked handshake, and the two wake-ups. Two more assert that a wake-up over a
healthy socket does nothing, and pass against either version on purpose: they
are what stops the cure being worse.

Left alone deliberately: a socket that is open and dead shows no banner at all,
because readyState still reads OPEN. That looks like a terminal that swallows
what is typed, needs a liveness probe rather than a faster retry, and is written
down at the end of 11.12a rather than quietly bundled in here.

docs/manual-checks.md

@@ -171,6 +171,32 @@ bound to the same side of `IsImportOpen`. SettingsNav lighting a different row w
 over the importer would be a guard added to `OnKeyDown` for `IsSettingsMode` that the design never asked for
 and this application's own quick-connect card was built to reach past.
 
+### 1.10 A terminal left alone for a long time is still a terminal · **needs an hour, or a debugger**
+
+Open a shell, leave the terminal for another screen — HOSTS, FILES, anything — and leave the application
+alone for long enough that the window has been in the background for the better part of an hour. Locking the
+machine or letting it sleep counts and is the easier way to get there. Come back and click the session's
+tab.
+
+**Pass:** the pane is exactly where it was and takes input straight away. If anything is shown at all it is
+`Reconnecting the terminal view…` for a moment, in the second or so before the socket is back — never a
+status that is still there after that.
+
+**Both of the page's own rules here are covered by `RendererReconnectionTests`**, which runs `terminal.js`
+itself in a fake browser — so a failure of this check is more likely to be the WebView behaving unlike that
+fake than the page's logic being wrong. That is exactly the division: the test owns the logic, this owns the
+platform.
+
+**Failure means:** a banner that stays up is the page's retry not running. It is a `setTimeout` chain, and a
+chain is what a WebView is entitled to throttle or freeze while nobody is looking at the page; `terminal.js`
+answers that with wake-ups on `visibilitychange`, `focus` and `online`, none of which can be throttled,
+plus a watchdog for a handshake that never finishes. A banner that flickers on and off every second or two
+instead is the opposite fault — a reconnect loop, in which each attempt displaces the socket before it
+through `TerminalDataPlane.UpgradeAsync`'s takeover and the displaced socket's close schedules the next.
+That is what the "is this still the page's socket" guard in `connect()` exists to stop. A pane that takes no
+input while the banner is *clear* is neither: the socket is open and dead, which nothing on this page can
+currently see — see the note at the end of 11.12a.
+
 ---
 
 ## Phase 2 — Known Hosts as its own page
@@ -1797,6 +1823,29 @@ worse than saying nothing: the banner exists so this is never silently wrong. If
 banner is there, the session's credit window was not reset on reattach and the shell is frozen behind it —
 see `TerminalWorkspace.ReplayAfterAttachAsync`.
 
+### 11.12a Coming back to a terminal screen left alone for a long while
+
+The same shape as 11.12 and a different trigger: rather than backgrounding the app, stay in it. With a shell
+open, leave the terminal for HOSTS, FILES or MORE — which collapses the renderer to GONE, so the page is
+hidden by Chromium's reckoning — and leave the phone alone for at least ten minutes with the screen off.
+Then come back to the app and to the terminal.
+
+**Pass:** the pane is there and takes input at once, or reconnects visibly within about a second of the
+screen appearing. `Reconnecting the terminal view…` on the way in is fine; still being there once the
+terminal has been on screen for a couple of seconds is not.
+
+**Failure means:** the page's retry did not survive being hidden. A hidden WebView has its timers throttled
+— once a minute after five minutes hidden — and a renderer that was frozen or reclaimed runs none of them,
+which is why `terminal.js` does not rely on the timer alone: `visibilitychange` is the event that says the
+screen is back, and it reconnects immediately rather than waiting to be asked twice. Ten minutes is chosen
+to clear the five-minute threshold with room to spare.
+
+**Not covered by either check, and worth knowing:** a socket that is *open and dead* — the connection gone
+without either end noticing, which a suspended renderer can leave behind — shows no banner at all, because
+`readyState` still reads OPEN and nothing on this page probes further. The symptom is a terminal that looks
+connected and swallows what is typed. If that is ever seen, it is a different bug from this one and needs a
+liveness probe rather than a faster retry.
+
 ---
 
 ## Phase 12 — Shared vaults: the operations that span two accounts