Keep the hour it is now on the axis

The visible range is drawn from the events in view, and in the evening the
events are behind you: the bounds end at six, everything after that is in the
trailing strip, and the now line has nowhere to land. The app stops saying
where in the day you are, which is most of what it is open for. Same at seven
in the morning, from the other end.

So when today is one of the columns, the axis takes that one hour in, and
everything the widening reached over folds behind it. At half eleven at night
the fixture week reads 7am to 8pm as usual, then an "8pm to 11pm" strip, then
one 11pm row with the line in it. A row and a strip, not four rows of empty
evening. Only the hour itself is opened, so how far the clock has drifted past
the last event costs nothing.

The pin sits on top of the hysteresis rather than inside it. previous.current
still holds what the events and the user asked for, so an hour pinned open
tonight cannot accumulate into the bounds the grid remembers tomorrow.

One consequence, and it is the only place the axis is allowed to move under
you: page to a week that does not contain today and the row goes again, along
with the strip it was sitting under.

The tick that drives it is an hour long, not a minute, because that is how
often the answer changes. It lives in a new useClock alongside the minute tick
the now line already had, which moves there out of GridNowLine. Both schedule
off the clock rather than off an interval, so a machine that was asleep
catches up on the next tick instead of drifting further out every hour.

The browser suite has to say what time it is now. The shape of the axis
depends on the hour a run happens at, so anything measuring a row height or
counting strips pins the clock to the middle of the working day, the same way
it already pins the theme and the week start. The three tests that are about
the clock ask for half eleven at night, which the fixture leaves empty.

tests/app.ts

@@ -23,8 +23,35 @@ export interface OpenOptions {
   view?: "day" | "week" | "agenda";
   /** Extra localStorage entries, written before the app's first script runs. */
   storage?: Record<string, string>;
+  /** Pins the browser's clock. See `clockAt`. */
+  now?: Date;
 }
 
+/**
+ * A fixed instant on the day the suite is being run, at a given hour of the pinned zone.
+ *
+ * The axis takes in the hour it is now, so the shape of the grid depends on when you run the
+ * suite: at eleven at night it carries a row and a strip that it does not carry at eleven in the
+ * morning. A test that measures the axis has to say which of those it means, the same way it says
+ * which theme and which week start it means. The one test that is *about* the clock asks for an
+ * hour the fixture leaves empty; every other one asks for the middle of the working day.
+ *
+ * Asia/Kolkata is UTC+5:30 the whole year round, so the offset can be written out.
+ */
+export function clockAt(hour: number, minute = 0): Date {
+  const day = new Intl.DateTimeFormat("en-CA", {
+    timeZone: "Asia/Kolkata",
+    year: "numeric",
+    month: "2-digit",
+    day: "2-digit",
+  }).format(new Date());
+  const pad = (n: number) => String(n).padStart(2, "0");
+  return new Date(`${day}T${pad(hour)}:${pad(minute)}:00+05:30`);
+}
+
+/** The middle of the working day: the fixture is busy, and the clock pins nothing extra open. */
+export const MIDDAY = () => clockAt(11, 30);
+
 const SEEDED = "__test-seeded";
 
 /**
@@ -56,6 +83,7 @@ export async function openApp(page: Page, options: OpenOptions = {}): Promise<vo
     { values: seed, flag: SEEDED },
   );
 
+  if (options.now) await page.clock.setFixedTime(options.now);
   await page.goto("/");
   if ((options.view ?? "week") === "agenda") await agendaReady(page);
   else await gridReady(page);