refactor(chat): drop the inert content-change and animation-handler contract

The old scroll engine needed message parts to report content growth
(onContentChange) and per-message animation lifecycle callbacks
(AnimationHandlers) so it could re-pin the viewport. The timeline list
measures growth itself now, and the replacement hook had already stubbed
the whole contract with no-ops kept only for source compatibility.

Remove it end to end: the hook exports, the container and list threading,
the ChatMessage/MessageBody signal-only effects, and every part-level
prop and call site. Expand/collapse behavior and reveal animations are
untouched — only the reporting channel goes.

packages/ui/src/components/chat/message/parts/AssistantTextPart.tsx

@@ -2,7 +2,6 @@ import React from 'react';
 import type { Part } from '@opencode-ai/sdk/v2';
 import { MarkdownRenderer } from '../../MarkdownRenderer';
 import type { StreamPhase, ToolPopupContent } from '../types';
-import type { ContentChangeReason } from '@/hooks/useChatTimelineScroll';
 import { useStreamingTextThrottle } from '../../hooks/useStreamingTextThrottle';
 import { resolveAssistantDisplayText, shouldRenderAssistantText } from './assistantTextVisibility';
 import { streamPerfCount, streamPerfObserve } from '@/stores/utils/streamDebug';
@@ -17,7 +16,6 @@ interface AssistantTextPartProps {
     messageId: string;
     streamPhase: StreamPhase;
     chatRenderMode?: 'sorted' | 'live';
-    onContentChange?: (reason?: ContentChangeReason, messageId?: string) => void;
     onShowPopup?: (content: ToolPopupContent) => void;
 }
 

packages/ui/src/components/chat/message/parts/JustificationBlock.tsx

@@ -1,6 +1,5 @@
 import React from 'react';
 import type { Part } from '@opencode-ai/sdk/v2';
-import type { ContentChangeReason } from '@/hooks/useChatTimelineScroll';
 import { useUIStore } from '@/stores/useUIStore';
 import { ReasoningTimelineBlock } from './ReasoningPart';
 
@@ -22,14 +21,12 @@ const cleanJustificationText = (text: string): string => {
 interface JustificationBlockProps {
     part: Part;
     messageId: string;
-    onContentChange?: (reason?: ContentChangeReason) => void;
     actions?: React.ReactNode;
 }
 
 const JustificationBlock: React.FC<JustificationBlockProps> = ({
     part,
     messageId,
-    onContentChange,
     actions,
 }) => {
     const chatRenderMode = useUIStore((state) => state.chatRenderMode);
@@ -47,7 +44,6 @@ const JustificationBlock: React.FC<JustificationBlockProps> = ({
         <ReasoningTimelineBlock
             text={textContent}
             variant="justification"
-            onContentChange={onContentChange}
             blockId={part.id || `${messageId}-justification`}
             time={time}
             showDuration={chatRenderMode !== 'sorted'}

packages/ui/src/components/chat/message/parts/ProgressiveGroup.tsx

@@ -4,7 +4,6 @@ import { cn } from '@/lib/utils';
 import type { TurnActivityRecord as TurnActivityPart } from '../../lib/turns/types';
 import type { ToolPart as ToolPartType } from '@opencode-ai/sdk/v2';
 import type { StreamPhase } from '../types';
-import type { ContentChangeReason } from '@/hooks/useChatTimelineScroll';
 import type { ToolPopupContent } from '../types';
 import ToolPart from './ToolPart';
 import { MinDurationShineText } from './MinDurationShineText';
@@ -40,7 +39,6 @@ interface ProgressiveGroupProps {
     expandedTools: Set<string>;
     onToggleTool: (toolId: string) => void;
     onShowPopup: (content: ToolPopupContent) => void;
-    onContentChange?: (reason?: ContentChangeReason) => void;
     streamPhase: StreamPhase;
     showHeader: boolean;
     animateRows?: boolean;
@@ -376,7 +374,6 @@ interface ExpandableToolRowProps {
     isMobile: boolean;
     onToggleTool: (toolId: string) => void;
     onShowPopup: (content: ToolPopupContent) => void;
-    onContentChange?: (reason?: ContentChangeReason) => void;
     animateTailText: boolean;
 }
 
@@ -386,7 +383,6 @@ const ExpandableToolRow: React.FC<ExpandableToolRowProps> = ({
     isMobile,
     onToggleTool,
     onShowPopup,
-    onContentChange,
     animateTailText,
 }) => {
     const handleToggle = React.useCallback(() => {
@@ -399,7 +395,6 @@ const ExpandableToolRow: React.FC<ExpandableToolRowProps> = ({
             isExpanded={isExpanded}
             onToggle={handleToggle}
             isMobile={isMobile}
-            onContentChange={onContentChange}
             onShowPopup={onShowPopup}
             animateTailText={animateTailText}
         />
@@ -423,7 +418,6 @@ const MemoExpandableToolRow = React.memo(ExpandableToolRow, (prev, next) => {
         && prev.isMobile === next.isMobile
         && prev.onToggleTool === next.onToggleTool
         && prev.onShowPopup === next.onShowPopup
-        && prev.onContentChange === next.onContentChange
         && prev.animateTailText === next.animateTailText
         && prev.activity.id === next.activity.id
         && prev.activity.kind === next.activity.kind
@@ -789,9 +783,8 @@ export const StaticToolRow = React.memo(StaticToolRowInner, (prev, next) => {
 /**
  * Inline reasoning text block — rendered as dimmed italic markdown.
  */
-const InlineReasoningBlock = React.memo(({ activity, onContentChange, streamPhase }: {
+const InlineReasoningBlock = React.memo(({ activity, streamPhase }: {
     activity: TurnActivityPart;
-    onContentChange?: (reason?: ContentChangeReason) => void;
     streamPhase: StreamPhase;
 }) => {
     return (
@@ -799,7 +792,6 @@ const InlineReasoningBlock = React.memo(({ activity, onContentChange, streamPhas
             part={activity.part}
             messageId={activity.messageId}
             streamPhase={streamPhase}
-            onContentChange={onContentChange}
         />
     );
 });
@@ -807,16 +799,14 @@ const InlineReasoningBlock = React.memo(({ activity, onContentChange, streamPhas
 /**
  * Inline justification text block — rendered as normal assistant text between tools.
  */
-const InlineJustificationBlock = React.memo(({ activity, onContentChange, actions }: {
+const InlineJustificationBlock = React.memo(({ activity, actions }: {
     activity: TurnActivityPart;
-    onContentChange?: (reason?: ContentChangeReason) => void;
     actions?: React.ReactNode;
 }) => {
     return (
         <JustificationBlock
             part={activity.part}
             messageId={activity.messageId}
-            onContentChange={onContentChange}
             actions={actions}
         />
     );
@@ -831,7 +821,6 @@ const ProgressiveGroup: React.FC<ProgressiveGroupProps> = ({
     expandedTools,
     onToggleTool,
     onShowPopup,
-    onContentChange,
     streamPhase,
     showHeader,
     animateRows = true,
@@ -892,7 +881,6 @@ const ProgressiveGroup: React.FC<ProgressiveGroupProps> = ({
                         <InlineReasoningBlock
                             activity={row.activity}
                             streamPhase={streamPhase}
-                            onContentChange={onContentChange}
                         />
                     </>
                 );
@@ -903,7 +891,6 @@ const ProgressiveGroup: React.FC<ProgressiveGroupProps> = ({
                     <>
                         <InlineJustificationBlock
                             activity={row.activity}
-                            onContentChange={onContentChange}
                             actions={renderJustificationActions?.(row.activity)}
                         />
                     </>
@@ -918,7 +905,6 @@ const ProgressiveGroup: React.FC<ProgressiveGroupProps> = ({
                         isMobile={isMobile}
                         onToggleTool={onToggleTool}
                         onShowPopup={onShowPopup}
-                        onContentChange={onContentChange}
                         animateTailText={Boolean(animatedToolIds?.has(row.activity.id))}
                     />
                 );
@@ -942,7 +928,6 @@ const ProgressiveGroup: React.FC<ProgressiveGroupProps> = ({
                         isMobile={isMobile}
                         onToggleTool={onToggleTool}
                         onShowPopup={onShowPopup}
-                        onContentChange={onContentChange}
                         animateTailText={Boolean(animatedToolIds?.has(row.activity.id))}
                     />
                 );

packages/ui/src/components/chat/message/parts/ReasoningPart.tsx

@@ -2,7 +2,6 @@ import React from 'react';
 import { animate, type AnimationPlaybackControls } from 'motion';
 import type { Part } from '@opencode-ai/sdk/v2';
 import { cn } from '@/lib/utils';
-import type { ContentChangeReason } from '@/hooks/useChatTimelineScroll';
 import { ScrollableOverlay } from '@/components/ui/ScrollableOverlay';
 import { Icon } from '@/components/icon/Icon';
 import { BusyDots } from './BusyDots';
@@ -81,7 +80,6 @@ const getReasoningSummary = (text: string): string => {
 type ReasoningTimelineBlockProps = {
     text: string;
     variant: ReasoningVariant;
-    onContentChange?: (reason?: ContentChangeReason) => void;
     blockId: string;
     time?: { start?: number; end?: number };
     showDuration?: boolean;
@@ -99,7 +97,6 @@ type ExpansionState = {
 export const ReasoningTimelineBlock: React.FC<ReasoningTimelineBlockProps> = ({
     text,
     variant,
-    onContentChange,
     blockId,
     time,
     isStreaming = false,
@@ -123,11 +120,6 @@ export const ReasoningTimelineBlock: React.FC<ReasoningTimelineBlockProps> = ({
     const contentRef = React.useRef<HTMLDivElement>(null);
     const contentAnimationRef = React.useRef<AnimationPlaybackControls | null>(null);
     const contentMountedRef = React.useRef(false);
-    // Stable handle to onContentChange so the height-animation layout effect can
-    // signal auto-follow without taking onContentChange as a dependency (which
-    // would risk re-running — and thus restarting — the animation on re-render).
-    const onContentChangeRef = React.useRef(onContentChange);
-    onContentChangeRef.current = onContentChange;
 
     const summary = React.useMemo(() => getReasoningSummary(text), [text]);
     const toggleAriaLabel = isExpanded
@@ -137,8 +129,7 @@ export const ReasoningTimelineBlock: React.FC<ReasoningTimelineBlockProps> = ({
     const handleToggle = React.useCallback(() => {
         setShouldRenderExpandedContent(true);
         setExpansion({ expanded: !isExpanded, source: 'user' });
-        onContentChange?.('structural');
-    }, [isExpanded, onContentChange]);
+    }, [isExpanded]);
 
     const handleKeyDown = React.useCallback((event: React.KeyboardEvent) => {
         if (event.key === 'Enter' || event.key === ' ') {
@@ -159,13 +150,6 @@ export const ReasoningTimelineBlock: React.FC<ReasoningTimelineBlockProps> = ({
         });
     }, [canAutoExpand]);
 
-    React.useEffect(() => {
-        if (text.trim().length === 0) {
-            return;
-        }
-        onContentChange?.('structural');
-    }, [onContentChange, text]);
-
     React.useEffect(() => {
         if (isExpanded || isStreaming) {
             setShouldRenderExpandedContent(true);
@@ -239,11 +223,6 @@ export const ReasoningTimelineBlock: React.FC<ReasoningTimelineBlockProps> = ({
             element.style.height = '0px';
         } else {
             element.style.height = `${element.scrollHeight}px`;
-            // Only the COLLAPSE animation needs the guard: it shrinks the
-            // timeline and the trailing async scroll events can be misread as a
-            // user scroll-away. Expansion grows the timeline and re-pins cleanly,
-            // and guarding it caused a faint scroll fight while thinking streams.
-            onContentChangeRef.current?.('animation');
         }
 
         const animation = animate(
@@ -436,14 +415,12 @@ export const ReasoningTimelineBlock: React.FC<ReasoningTimelineBlockProps> = ({
 
 type ReasoningPartProps = {
     part: Part;
-    onContentChange?: (reason?: ContentChangeReason) => void;
     messageId: string;
     streamPhase?: StreamPhase;
 };
 
 const ReasoningPart = React.memo(({
     part,
-    onContentChange,
     messageId,
     streamPhase,
 }: ReasoningPartProps) => {
@@ -470,7 +447,6 @@ const ReasoningPart = React.memo(({
         <ReasoningTimelineBlock
             text={throttledText}
             variant="thinking"
-            onContentChange={onContentChange}
             blockId={part.id || `${messageId}-reasoning`}
             time={time}
             isStreaming={isStreaming}

packages/ui/src/components/chat/message/parts/ToolPart.tsx

@@ -20,7 +20,6 @@ import { toast } from '@/components/ui';
 import { Text } from '@/components/ui/text';
 import { FileTypeIcon } from '@/components/icons/FileTypeIcon';
 import { copyTextToClipboard } from '@/lib/clipboard';
-import type { ContentChangeReason } from '@/hooks/useChatTimelineScroll';
 import type { ToolPopupContent } from '../types';
 import { PlainDiffFallback } from './PlainDiffFallback';
 import { lazyWithChunkRecovery } from '@/lib/chunkLoadRecovery';
@@ -82,7 +81,6 @@ interface ToolPartProps {
     onToggle: (toolId: string) => void;
     isMobile: boolean;
     alwaysShowActions?: boolean;
-    onContentChange?: (reason?: ContentChangeReason) => void;
     onShowPopup?: (content: ToolPopupContent) => void;
     animateTailText?: boolean;
 }
@@ -1684,7 +1682,6 @@ const ToolPartContent: React.FC<ToolPartProps> = ({
     isExpanded,
     onToggle,
     isMobile,
-    onContentChange,
     onShowPopup,
     animateTailText = true,
 }) => {
@@ -1754,10 +1751,6 @@ const ToolPartContent: React.FC<ToolPartProps> = ({
         });
     }, [currentDirectory, input, isFinalized, isSuccessfullyFinalized, metadata, normalizedPartTool]);
 
-    const shouldNotifyStructuralChange = isFinalized || isTaskTool;
-
-    const onContentChangeRef = React.useRef(onContentChange);
-    onContentChangeRef.current = onContentChange;
     const expandedContentRef = React.useRef<HTMLDivElement>(null);
 
     React.useLayoutEffect(() => {
@@ -1772,11 +1765,7 @@ const ToolPartContent: React.FC<ToolPartProps> = ({
 
         element.style.height = isExpanded ? 'auto' : '0px';
         element.style.overflow = isExpanded ? 'visible' : 'hidden';
-
-        if (shouldNotifyStructuralChange) {
-            onContentChangeRef.current?.('structural');
-        }
-    }, [isExpanded, isTaskTool, shouldNotifyStructuralChange]);
+    }, [isExpanded, isTaskTool]);
 
     const partMetadata = (part as unknown as { metadata?: unknown }).metadata;
     const time = stateWithData.time;
@@ -1934,26 +1923,6 @@ const ToolPartContent: React.FC<ToolPartProps> = ({
         }
         return metadataTaskSummaryEntries;
     }, [childSessionTaskSummaryEntries, metadataTaskSummaryEntries]);
-    const taskSummaryRenderSignature = React.useMemo(() => {
-        return taskSummaryEntries.map(getTaskSummaryEntryRenderSignature).join('\u0000');
-    }, [taskSummaryEntries]);
-    const lastTaskSummaryRenderSignatureRef = React.useRef<string | null>(null);
-
-    React.useEffect(() => {
-        if (!isTaskTool) {
-            lastTaskSummaryRenderSignatureRef.current = null;
-            return;
-        }
-
-        const previous = lastTaskSummaryRenderSignatureRef.current;
-        lastTaskSummaryRenderSignatureRef.current = taskSummaryRenderSignature;
-        if (previous === null || previous === taskSummaryRenderSignature || taskSummaryEntries.length === 0) {
-            return;
-        }
-
-        onContentChangeRef.current?.('structural');
-    }, [isTaskTool, taskSummaryEntries.length, taskSummaryRenderSignature]);
-
     const diffStats = React.useMemo(() => {
         return (normalizedPartTool === 'edit' || normalizedPartTool === 'multiedit' || normalizedPartTool === 'apply_patch')
             ? parseDiffStats(metadata)
@@ -2351,7 +2320,6 @@ export default React.memo(ToolPart, (prev, next) => {
         && prev.isExpanded === next.isExpanded
         && prev.isMobile === next.isMobile
         && prev.alwaysShowActions === next.alwaysShowActions
-        && prev.onContentChange === next.onContentChange
         && prev.onShowPopup === next.onShowPopup
         && prev.animateTailText === next.animateTailText;
 });