feat: sync timeline highlight with scroll

packages/ui/src/components/message-section.tsx

@@ -118,6 +118,7 @@ export default function MessageSection(props: MessageSectionProps) {
   })
  
   const hasTimelineSegments = () => timelineSegments().length > 0
+  const [activeMessageId, setActiveMessageId] = createSignal<string | null>(null)
  
   const changeToken = createMemo(() => String(sessionRevision()))
 
@@ -353,9 +354,44 @@ export default function MessageSection(props: MessageSectionProps) {
     observer.observe(bottomTarget)
     onCleanup(() => observer.disconnect())
   })
-
+ 
+  createEffect(() => {
+    const container = scrollElement()
+    const ids = messageIds()
+    if (!container || ids.length === 0) return
+    if (typeof document === "undefined") return
+ 
+    const observer = new IntersectionObserver(
+      (entries) => {
+        let best: IntersectionObserverEntry | null = null
+        for (const entry of entries) {
+          if (!entry.isIntersecting) continue
+          if (!best || entry.boundingClientRect.top < best.boundingClientRect.top) {
+            best = entry
+          }
+        }
+        if (best) {
+          const anchorId = (best.target as HTMLElement).id
+          const messageId = anchorId.startsWith("message-anchor-") ? anchorId.slice("message-anchor-".length) : anchorId
+          setActiveMessageId((current) => (current === messageId ? current : messageId))
+        }
+      },
+      { root: container, rootMargin: "-10% 0px -80% 0px", threshold: 0 },
+    )
+ 
+    ids.forEach((messageId) => {
+      const anchor = document.getElementById(getMessageAnchorId(messageId))
+      if (anchor) {
+        observer.observe(anchor)
+      }
+    })
+ 
+    onCleanup(() => observer.disconnect())
+  })
+ 
   onCleanup(() => {
 
+
     if (pendingScrollFrame !== null) {
       cancelAnimationFrame(pendingScrollFrame)
     }
@@ -462,7 +498,11 @@ export default function MessageSection(props: MessageSectionProps) {
  
         <Show when={hasTimelineSegments()}>
           <div class="message-timeline-sidebar">
-            <MessageTimeline segments={timelineSegments()} onSegmentClick={handleTimelineSegmentClick} />
+            <MessageTimeline
+              segments={timelineSegments()}
+              onSegmentClick={handleTimelineSegmentClick}
+              activeMessageId={activeMessageId()}
+            />
           </div>
         </Show>
       </div>

packages/ui/src/components/message-timeline.tsx

@@ -1,4 +1,4 @@
-import { For, type Component } from "solid-js"
+import { For, createEffect, onCleanup, type Component } from "solid-js"
 import type { ClientPart } from "../types/message"
 import type { MessageRecord } from "../stores/message-v2/types"
 import { buildRecordDisplayData } from "../stores/message-v2/record-display-cache"
@@ -16,6 +16,7 @@ export interface TimelineSegment {
 interface MessageTimelineProps {
   segments: TimelineSegment[]
   onSegmentClick?: (segment: TimelineSegment) => void
+  activeMessageId?: string | null
 }
 
 const SEGMENT_LABELS: Record<TimelineSegmentType, string> = {
@@ -202,23 +203,57 @@ export function buildTimelineSegments(instanceId: string, record: MessageRecord)
 }
 
 const MessageTimeline: Component<MessageTimelineProps> = (props) => {
+  const buttonRefs = new Map<string, HTMLButtonElement>()
+ 
+  const registerButtonRef = (segmentId: string, element: HTMLButtonElement | null) => {
+    if (element) {
+      buttonRefs.set(segmentId, element)
+    } else {
+      buttonRefs.delete(segmentId)
+    }
+  }
+ 
+  createEffect(() => {
+    const activeId = props.activeMessageId
+    if (!activeId) return
+    const targetSegment = props.segments.find((segment) => segment.messageId === activeId)
+    if (!targetSegment) return
+    const element = buttonRefs.get(targetSegment.id)
+    if (!element) return
+    const timer = typeof window !== "undefined" ? window.setTimeout(() => {
+      element.scrollIntoView({ block: "nearest", behavior: "smooth" })
+    }, 120) : null
+    onCleanup(() => {
+      if (timer !== null && typeof window !== "undefined") {
+        window.clearTimeout(timer)
+      }
+    })
+  })
+ 
   return (
     <div class="message-timeline" role="navigation" aria-label="Message timeline">
       <For each={props.segments}>
-        {(segment) => (
-          <button
-            type="button"
-            class={`message-timeline-segment message-timeline-${segment.type}`}
-            title={segment.tooltip}
-            onClick={() => props.onSegmentClick?.(segment)}
-          >
-            <span class="message-timeline-label message-timeline-label-full">{segment.label}</span>
-            <span class="message-timeline-label message-timeline-label-short">{SEGMENT_SHORT_LABELS[segment.type]}</span>
-          </button>
-        )}
+        {(segment) => {
+          onCleanup(() => buttonRefs.delete(segment.id))
+          const isActive = () => props.activeMessageId === segment.messageId
+          return (
+            <button
+              ref={(el) => registerButtonRef(segment.id, el)}
+              type="button"
+              class={`message-timeline-segment message-timeline-${segment.type} ${isActive() ? "message-timeline-segment-active" : ""}`}
+              title={segment.tooltip}
+              aria-current={isActive() ? "true" : undefined}
+              onClick={() => props.onSegmentClick?.(segment)}
+            >
+              <span class="message-timeline-label message-timeline-label-full">{segment.label}</span>
+              <span class="message-timeline-label message-timeline-label-short">{SEGMENT_SHORT_LABELS[segment.type]}</span>
+            </button>
+          )
+        }}
       </For>
     </div>
   )
 }
 
+
 export default MessageTimeline

packages/ui/src/styles/messaging/message-timeline.css

@@ -1,7 +1,7 @@
 .message-layout {
   display: grid;
   grid-template-columns: minmax(0, 1fr);
-  gap: 0.5rem;
+  gap: 0rem;
   width: 100%;
   min-height: 0;
   flex: 1 1 auto;
@@ -77,7 +77,15 @@
   letter-spacing: 0.05em;
   text-transform: uppercase;
   color: var(--text-primary);
-  transition: transform 0.15s ease, background-color 0.2s ease, color 0.2s ease;
+  transition: transform 0.15s ease, background-color 0.2s ease, color 0.2s ease, border-color 0.2s ease, box-shadow 0.2s ease;
+}
+
+.message-timeline-segment-active {
+  border-color: transparent;
+  background-color: #0f5b44;
+  color: #fff;
+  font-weight: 700;
+  box-shadow: inset 0 0 0 1px rgba(0, 0, 0, 0.35);
 }
 
 .message-timeline-segment:hover,
@@ -88,6 +96,15 @@
   transform: translateY(-1px);
 }
 
+.message-timeline-segment-active,
+.message-timeline-segment-active:hover,
+.message-timeline-segment-active:focus-visible {
+  background-color: #0f5b44;
+  color: #fff;
+  transform: none;
+  box-shadow: inset 0 0 0 1px rgba(0, 0, 0, 0.35);
+}
+
 .message-timeline-segment:focus-visible {
   box-shadow: 0 0 0 2px var(--surface-base), 0 0 0 4px var(--accent-primary);
 }
@@ -107,6 +124,14 @@
   background-color: var(--surface-secondary);
 }
 
+.message-timeline-segment-active {
+  background-color: #0f5b44 !important;
+  border-color: transparent !important;
+  color: #fff !important;
+  font-weight: 700;
+  box-shadow: inset 0 0 0 1px rgba(0, 0, 0, 0.35);
+}
+
 .message-timeline-label {
   pointer-events: none;
 }