fix(weather-map): default timeline cursor to the hour closest to now

Previously the map loaded the latest cached valid_time from GridCache,
which can be any +Nh forecast hour the worker has written while walking
f00-f18. Users expect to see current conditions on load, not a forecast
that's ~11 hours out.

lib/microwaveprop_web/live/weather_map_live.ex

@@ -143,14 +143,25 @@ defmodule MicrowavepropWeb.WeatherMapLive do
       Phoenix.PubSub.subscribe(Microwaveprop.PubSub, "propagation:pipeline")
     end
 
-    # Hot-path mount: use the cache-only latest_weather_grid for the
-    # initial payload so mount stays under the LV socket join timeout
-    # (~10s). Reading and deriving the full 92k-point ProfilesFile
-    # synchronously pushes mount past that window on a cold pod,
-    # which makes the LV client fall back to a full page reload.
-    data = Weather.latest_weather_grid(@initial_bounds)
-    latest_vt = if data == [], do: Weather.latest_grid_valid_time(), else: hd(data).valid_time
+    # Default the timeline cursor to the valid_time closest to "now" so
+    # users land on current conditions, not a +11h forecast hour (which
+    # the underlying cache can advance to while the worker is walking
+    # f00→f18).
+    #
+    # Hot-path mount: when the closest-to-now time happens to be the
+    # cached latest, use `latest_weather_grid/1` so mount stays under
+    # the LV socket join timeout (~10s). A ProfilesFile read for any
+    # other hour is a single ~2 MB ETF decode, also fine for mount.
     valid_times = Weather.available_weather_valid_times()
+    latest_vt = Weather.latest_grid_valid_time()
+    initial_vt = pick_initial_valid_time(valid_times)
+
+    data =
+      cond do
+        is_nil(initial_vt) -> []
+        initial_vt == latest_vt -> Weather.latest_weather_grid(@initial_bounds)
+        true -> Weather.weather_grid_at(initial_vt, @initial_bounds)
+      end
 
     {:ok,
      assign(socket,
@@ -158,10 +169,10 @@ defmodule MicrowavepropWeb.WeatherMapLive do
        layers: @layers,
        selected_layer: "refractivity_gradient",
        initial_data_json: Jason.encode!(data),
-       valid_time: latest_vt,
+       valid_time: initial_vt,
        valid_times: valid_times,
        initial_valid_times_json: Jason.encode!(Enum.map(valid_times, &DateTime.to_iso8601/1)),
-       initial_selected_time: latest_vt && DateTime.to_iso8601(latest_vt),
+       initial_selected_time: initial_vt && DateTime.to_iso8601(initial_vt),
        initial_utc_clock: Calendar.strftime(DateTime.utc_now(), "%H:%M UTC"),
        bounds: @initial_bounds,
        grid_visible: false,
@@ -169,6 +180,13 @@ defmodule MicrowavepropWeb.WeatherMapLive do
      )}
   end
 
+  defp pick_initial_valid_time([]), do: nil
+
+  defp pick_initial_valid_time(valid_times) do
+    now = DateTime.utc_now()
+    Enum.min_by(valid_times, fn vt -> abs(DateTime.diff(vt, now, :second)) end)
+  end
+
   @impl true
   def handle_event("select_layer", %{"layer" => layer_id}, socket) do
     {:noreply, assign(socket, :selected_layer, layer_id)}

test/microwaveprop_web/live/weather_map_live_test.exs

@@ -75,6 +75,33 @@ defmodule MicrowavepropWeb.WeatherMapLiveTest do
       end)
     end
 
+    test "mount defaults to the valid_time closest to now, not the latest forecast hour",
+         %{conn: conn} do
+      base =
+        DateTime.utc_now()
+        |> DateTime.truncate(:second)
+        |> Map.put(:minute, 0)
+        |> Map.put(:second, 0)
+
+      # Seed f00 (now) and f11 (+11h). The worker walks forecast hours
+      # sequentially so `latest_grid_valid_time` can point to f11 even
+      # though the user expects the map to show "Now" by default.
+      now_t = base
+      future_t = DateTime.add(base, 11 * 3600, :second)
+
+      write_profile(now_t, 33.0, -97.0)
+      write_profile(future_t, 33.0, -97.0)
+
+      {:ok, _lv, html} = live(conn, ~p"/weather")
+
+      now_iso = DateTime.to_iso8601(now_t)
+      future_iso = DateTime.to_iso8601(future_t)
+
+      # The map element's data-selected-time attribute should be f00, not f11.
+      assert html =~ ~s(data-selected-time="#{now_iso}")
+      refute html =~ ~s(data-selected-time="#{future_iso}")
+    end
+
     test "select_time replaces the rendered weather data with the new hour's grid", %{conn: conn} do
       base =
         DateTime.utc_now()