Anchor log viewer scroll to visible content and add 1m/2m timeline presets

The log table no longer jumps when a sliding window prunes rows from the
front: scroll position now tracks the same visible row instead of holding
scrollTop constant in pixels. Also adds 1m/2m to the timeline quick-range
presets.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01XFUNZwCvz3kcod3352ZR2o

webapp/src/components/LogPreview.tsx

@@ -82,9 +82,51 @@ function ColumnPicker({
   )
 }
 
+/// Keeps the scroll position anchored to the same visible row across a `logs` update (e.g. a sliding time window
+/// pruning old rows from the front), instead of the browser's default of holding `scrollTop` constant in pixels —
+/// which would otherwise scroll the view to different content whenever rows are removed above it.
+function useScrollAnchor(containerRef: React.RefObject<HTMLDivElement | null>, deps: unknown[]) {
+  const pendingAnchor = useRef<{ key: string; offsetTop: number } | null>(null)
+
+  useEffect(() => {
+    const container = containerRef.current
+    if (!container) return
+
+    if (pendingAnchor.current) {
+      const { key, offsetTop } = pendingAnchor.current
+      const row = container.querySelector(`[data-row-key="${CSS.escape(key)}"]`)
+      if (row) {
+        const rowTop = row.getBoundingClientRect().top - container.getBoundingClientRect().top
+        container.scrollTop += rowTop - offsetTop
+      }
+      pendingAnchor.current = null
+    }
+
+    // Runs right before the DOM mutates for the next update (React calls a layout effect's cleanup
+    // synchronously before re-running it), so this still sees the pre-update DOM — the same idea as
+    // `getSnapshotBeforeUpdate`, expressed with a plain effect.
+    return () => {
+      const containerRect = container.getBoundingClientRect()
+      for (const row of container.querySelectorAll('[data-row-key]')) {
+        const rect = row.getBoundingClientRect()
+        if (rect.bottom > containerRect.top) {
+          pendingAnchor.current = {
+            key: row.getAttribute('data-row-key')!,
+            offsetTop: rect.top - containerRect.top,
+          }
+          break
+        }
+      }
+    }
+    // eslint-disable-next-line react-hooks/exhaustive-deps
+  }, deps)
+}
+
 export function LogPreview({ logs, params }: { logs: LogEntry[]; params: ParamDeclaration[] }) {
   const paramNames = params.map((d) => d.name)
   const [visibleParamColumns, setVisibleParamColumns] = useState<string[]>(loadVisibleParamColumns)
+  const scrollContainerRef = useRef<HTMLDivElement>(null)
+  useScrollAnchor(scrollContainerRef, [logs])
 
   const visible = new Set(visibleParamColumns)
 
@@ -110,7 +152,7 @@ export function LogPreview({ logs, params }: { logs: LogEntry[]; params: ParamDe
         </div>
         <ColumnPicker paramNames={paramNames} visible={visible} onToggle={toggleColumn} />
       </div>
-      <div className="flex-1 overflow-auto">
+      <div ref={scrollContainerRef} className="flex-1 overflow-auto">
         <table className="w-full border-collapse text-left text-xs">
           <thead className="sticky top-0 bg-surface-0 text-text-subtle">
             <tr>
@@ -126,7 +168,7 @@ export function LogPreview({ logs, params }: { logs: LogEntry[]; params: ParamDe
           </thead>
           <tbody>
             {logs.map((log) => (
-              <tr key={log.sequence} className="border-t border-border-faint hover:bg-surface-1/60">
+              <tr key={log.sequence} data-row-key={log.sequence} className="border-t border-border-faint hover:bg-surface-1/60">
                 <td className="whitespace-nowrap px-3 py-1 text-text-subtle">{log.sequence}</td>
                 <td className="whitespace-nowrap px-3 py-1 text-text-subtle">
                   {new Date(log.timestamp_ms).toLocaleTimeString()}

webapp/src/components/LogTimeline.tsx

@@ -9,10 +9,12 @@ export interface TimeRange {
 }
 
 // `'all'` clears the range (the implicit default); the rest resolve to a window ending "now" at the moment they're (re-)computed — see `computePresetRange`.
-export type TimeRangePresetId = 'all' | '5m' | '15m' | '1h' | '24h'
+export type TimeRangePresetId = 'all' | '1m' | '2m' | '5m' | '15m' | '1h' | '24h'
 
 export const TIME_RANGE_PRESETS: { id: TimeRangePresetId; label: string; ms: number | null }[] = [
   { id: 'all', label: 'All', ms: null },
+  { id: '1m', label: '1m', ms: 60 * 1000 },
+  { id: '2m', label: '2m', ms: 2 * 60 * 1000 },
   { id: '5m', label: '5m', ms: 5 * 60 * 1000 },
   { id: '15m', label: '15m', ms: 15 * 60 * 1000 },
   { id: '1h', label: '1h', ms: 60 * 60 * 1000 },