Calendar · open times start over on new days

AvailabilityPicker kept the day chosen in keyed state whatever days the
owner passed next, and the slots lookup expected it among them: an
owner paging to the next week crashed the app once a day was chosen.
The choice is now seeded on the owner's days and pick, so a new run of
days starts it over at the picked day or the first open one.

TASKS.md

@@ -358,3 +358,4 @@ Per-component lines live in `tasks/`. Tags there:
 | T30b | 1 | FAIL | the week and day views froze the line at now and today: the moment read once went down as given and turned the ticker off |
 | T30b | 2 | PASS | |
 | T30c | 1 | PASS | |
+| T30d | 1 | FAIL | a chosen day outlived the owner's days, and the next render panicked |

src/calendar/availability.rs

@@ -8,6 +8,7 @@ use jiff::{Timestamp, civil::Date, tz::TimeZone};
 use super::hours::moment;
 use crate::{
     forms::{Choice, ChoiceChips},
+    layout::seeded::use_seeded,
     theme::{ActiveTheme, TextSize},
     typography::format,
 };
@@ -41,7 +42,7 @@ pub(crate) fn open_slots(
     starts
 }
 
-/// Open times to meet over some days: each day with its count of open slots, then the slots of the one chosen. Busy stretches, the working hours and the past close the rest. A pick hands on the slot's start.
+/// Open times to meet over some days: each day with its count of open slots, then the slots of the one chosen. Busy stretches, the working hours and the past close the rest. A pick hands on the slot's start; new days from the owner start the choice over.
 #[derive(IntoElement)]
 pub struct AvailabilityPicker {
     id: ElementId,
@@ -149,8 +150,9 @@ impl RenderOnce for AvailabilityPicker {
             .iter()
             .find(|(_, slots)| !slots.is_empty())
             .map(|(day, _)| *day);
-        let chosen = window.use_keyed_state((self.id.clone(), "day"), cx, |_, _| None::<Date>);
-        let shown = chosen.read(cx).or(picked_day).or(first_open);
+        let seed = (self.days.clone(), picked_day.or(first_open));
+        let chosen = use_seeded((self.id.clone(), "day"), seed, window, cx);
+        let shown = chosen.read(cx).value.1;
         let theme = cx.theme();
         let quiet = |text: &'static str| {
             div()
@@ -184,7 +186,7 @@ impl RenderOnce for AvailabilityPicker {
                 .expect("a day's value is its date");
             log::info!("availability picker: {day}");
             chosen.update(cx, |chosen, cx| {
-                *chosen = Some(day);
+                chosen.value.1 = Some(day);
                 cx.notify();
             })
         });

src/calendar/tests/editing.rs

@@ -161,3 +161,34 @@ fn a_slot_pick_hands_on_its_start(cx: &mut TestAppContext) {
         "09:00 to 10:00 is taken"
     );
 }
+
+fn paged(host: &Planning, owner: Entity<Planning>) -> gpui::AnyElement {
+    let first = host.month;
+    let dawn = date(2026, 8, 31).at(0, 0, 0, 0).to_zoned(TimeZone::UTC);
+    AvailabilityPicker::new("paged", [first, first.tomorrow().expect("a day ahead")])
+        .zone(TimeZone::UTC)
+        .hours(9, 10)
+        .now(dawn.expect("a UTC time").timestamp())
+        .on_pick(move |start, _, cx| note(&owner, format!("pick {start}"), cx))
+        .into_any_element()
+}
+
+#[gpui::test]
+fn new_days_from_the_owner_start_the_choice_over(cx: &mut TestAppContext) {
+    let (host, cx) = planning(paged, cx);
+    tab_to(2, cx);
+    press("space", cx);
+    host.update(cx, |host, cx| {
+        host.month = date(2026, 10, 5);
+        cx.notify();
+    });
+    settle(cx);
+    tab_to(3, cx);
+    press("space", cx);
+    let nine = date(2026, 10, 5).at(9, 0, 0, 0).to_zoned(TimeZone::UTC);
+    assert_eq!(
+        heard(&host, cx),
+        [format!("pick {}", nine.expect("a UTC time").timestamp())],
+        "the chosen day of the old run is gone; the first open day shows"
+    );
+}