fix(timeline): pre-load 90-day past buffer + scroll to today on mount

viewStart=today left no canvas to the left of scrollLeft=0, making
left-scroll physically impossible. Now viewStart defaults to today-90
so the canvas always has 90 days to scroll into, and a mount-time
useLayoutEffect positions the viewport with today at the left edge.

The Today button restores this view: scrolls in-range, or resets
viewStart and schedules a post-layout scroll if today has scrolled
out of the visible window.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

apps/web/src/components/timeline/TimelineContext.tsx

@@ -284,7 +284,7 @@ export function TimelineProvider({
       const d = new Date(sp);
       if (!isNaN(d.getTime())) return d;
     }
-    return today;
+    return addDays(today, -90);
   });
   const [viewDays, setViewDays] = useState(() => {
     const sp = searchParams.get("days");
@@ -310,7 +310,7 @@ export function TimelineProvider({
         const d = new Date(spStart);
         if (!isNaN(d.getTime())) return d;
       }
-      return today;
+      return addDays(today, -90);
     });
 
     const spDays = searchParams.get("days");

apps/web/src/components/timeline/TimelineView.tsx

@@ -687,15 +687,33 @@ function TimelineViewContent({
 
   // Pixels to add to scrollLeft after a left-extension re-render (prevents jump).
   const pendingLeftCompensationPx = useRef(0);
+  // Flag: scroll viewport to today after the next viewStart-driven re-layout.
+  const pendingScrollToTodayRef = useRef(false);
+  // Guard: only auto-scroll to today once on initial mount.
+  const scrolledToTodayOnMount = useRef(false);
 
-  // Apply scroll compensation synchronously after the canvas grows leftward.
+  // Scroll to today on first mount so the viewport opens with today at the left edge.
   useLayoutEffect(() => {
-    const px = pendingLeftCompensationPx.current;
-    if (px === 0) return;
+    if (scrolledToTodayOnMount.current) return;
     const el = scrollContainerRef.current;
-    if (el) el.scrollLeft += px;
+    if (!el) return;
+    el.scrollLeft = toLeft(today);
+    scrolledToTodayOnMount.current = true;
+  }, [toLeft, today]);
+
+  // Apply scroll compensation synchronously after the canvas grows (left-extend or Today button).
+  useLayoutEffect(() => {
+    const el = scrollContainerRef.current;
+    if (!el) return;
+    const px = pendingLeftCompensationPx.current;
+    if (px !== 0) {
+      el.scrollLeft += px;
       pendingLeftCompensationPx.current = 0;
-  }, [viewStart]);
+    } else if (pendingScrollToTodayRef.current) {
+      el.scrollLeft = toLeft(today);
+      pendingScrollToTodayRef.current = false;
+    }
+  }, [viewStart, toLeft, today]);
 
   // 5-year floor — no practical data exists further back; prevents runaway growth.
   const minDate = useMemo(() => addDays(today, -(365 * 5)), [today]);
@@ -709,7 +727,20 @@ function TimelineViewContent({
       }),
     [setViewStart, minDate],
   );
-  const handleNavigateToday = useCallback(() => setViewStart(today), [setViewStart, today]);
+  const handleNavigateToday = useCallback(() => {
+    const el = scrollContainerRef.current;
+    const todayMs = new Date(today).setHours(0, 0, 0, 0);
+    const vsMs = new Date(viewStart).setHours(0, 0, 0, 0);
+    const veMs = new Date(addDays(viewStart, viewDays)).setHours(0, 0, 0, 0);
+    if (todayMs >= vsMs && todayMs < veMs && el) {
+      // Today is in range — just scroll without touching state.
+      el.scrollLeft = toLeft(today);
+    } else {
+      // Today is out of range — reset the window and schedule a scroll.
+      pendingScrollToTodayRef.current = true;
+      setViewStart(addDays(today, -90));
+    }
+  }, [today, viewStart, viewDays, toLeft, setViewStart]);
   const handleNavigateForward = useCallback(
     () => setViewStart((v) => addDays(v, 28)),
     [setViewStart],