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Merged
urjitc merged 3 commits into from
Aug 15, 2026
Merged

fix(ui): keep the transcript still when the composer resizes #790

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55a3699
fix(workspaces): drop the assistant status row once the reply renders
urjitc Aug 15, 2026
dbe4ae8
fix(ui): keep the transcript still when the composer resizes
urjitc Aug 15, 2026
5a5e289
docs(ui): name the positions the scroller actually restores
urjitc Aug 15, 2026
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119 changes: 38 additions & 81 deletions src/components/ui/message-scroller-primitive/components.tsx
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Original file line number Diff line number Diff line change
Expand Up @@ -10,8 +10,7 @@ import type {
MessageScrollerProviderProps,
MessageScrollerViewportProps,
} from "./types";
import { useMessageScrollerController } from "./use-message-scroller-controller";
import { composeRefs, useLatest } from "./utils";
import { useMessageScroller } from "./use-message-scroller";

const MessageScrollerContext = React.createContext<MessageScrollerContextValue | null>(null);

Expand All @@ -25,44 +24,31 @@ function useMessageScrollerContext() {
return context;
}

function applyRef<T>(ref: React.Ref<T> | undefined, value: T | null) {
if (typeof ref === "function") {
ref(value);

@cubic-dev-ai cubic-dev-ai Bot Aug 15, 2026

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P2: When a consumer passes a React 19 callback ref with a cleanup return to MessageScrollerViewport or MessageScrollerContent, applyRef discards the return value. Because the wrapper refs are the refs React sees, React never runs that cleanup on unmount or ref replacement; preserve and compose the returned cleanup while clearing the scroller element.

Prompt for AI agents 
Check if this issue is valid — if so, understand the root cause and fix it. At src/components/ui/message-scroller-primitive/components.tsx, line 29:

<comment>When a consumer passes a React 19 callback ref with a cleanup return to `MessageScrollerViewport` or `MessageScrollerContent`, `applyRef` discards the return value. Because the wrapper refs are the refs React sees, React never runs that cleanup on unmount or ref replacement; preserve and compose the returned cleanup while clearing the scroller element.</comment>

<file context>
@@ -25,44 +24,31 @@ function useMessageScrollerContext() {
 
+function applyRef<T>(ref: React.Ref<T> | undefined, value: T | null) {
+	if (typeof ref === "function") {
+		ref(value);
+	} else if (ref) {
+		ref.current = value;
</file context>

} else if (ref) {
ref.current = value;
}
}

function MessageScrollerProvider({
appendedAnchorScrollBehavior,
autoScroll = false,
children,
defaultScrollPosition = "end",
scrollEdgeThreshold,
scrollPreviousItemPeek,
scrollMargin,
}: MessageScrollerProviderProps) {
const { context } = useMessageScrollerController({
const context = useMessageScroller({
appendedAnchorScrollBehavior,
autoScroll,
defaultScrollPosition,
scrollEdgeThreshold,
scrollPreviousItemPeek,
scrollMargin,
});

return (
<MessageScrollerContext.Provider value={context}>{children}</MessageScrollerContext.Provider>
);
}

function MessageScroller({ children, ref, ...props }: MessageScrollerProps) {
const { setRootElement } = useMessageScrollerContext();
const setRootRef = React.useCallback(
(element: HTMLDivElement | null) => {
setRootElement(element);
composeRefs(ref)?.(element);
},
[ref, setRootElement],
);

return (
<div {...props} ref={setRootRef}>
{children}
</div>
);
function MessageScroller(props: MessageScrollerProps) {
return <div {...props} />;
}

function MessageScrollerViewport({
Expand All @@ -73,30 +59,18 @@ function MessageScrollerViewport({
onScroll,
onTouchMove,
onWheel,
preserveScrollOnPrepend = true,
ref,
role,
tabIndex,
...props
}: MessageScrollerViewportProps) {
const {
handleResize,
preserveScrollOnPrependRef,
setViewportElement,
syncAfterScroll,
userScrollIntent,
viewportRef,
} = useMessageScrollerContext();
const { handleResize, setViewportElement, syncAfterScroll, userScrollIntent, viewportRef } =
useMessageScrollerContext();
const pointerScrollIntentRef = React.useRef(false);

React.useLayoutEffect(() => {
preserveScrollOnPrependRef.current = preserveScrollOnPrepend;
}, [preserveScrollOnPrepend, preserveScrollOnPrependRef]);

const setViewportRef = React.useCallback(
(element: HTMLDivElement | null) => {
setViewportElement(element);
composeRefs(ref)?.(element);
applyRef(ref, element);
},
[ref, setViewportElement],
);
Expand Down Expand Up @@ -181,12 +155,11 @@ function MessageScrollerContent({
const { handleContentChange, handleResize, setContentElement, setSpacerElement } =
useMessageScrollerContext();
const contentRef = React.useRef<HTMLDivElement | null>(null);

const setContentRef = React.useCallback(
(element: HTMLDivElement | null) => {
contentRef.current = element;
setContentElement(element);
composeRefs(ref)?.(element);
applyRef(ref, element);
},
[ref, setContentElement],
);
Expand All @@ -204,9 +177,7 @@ function MessageScrollerContent({
return;
}

const observer = new MutationObserver(() => {
handleContentChange();
});
const observer = new MutationObserver(handleContentChange);

observer.observe(content, { childList: true });

Expand Down Expand Up @@ -265,57 +236,43 @@ function MessageScrollerItem({
function MessageScrollerButton({
behavior = "smooth",
children,
direction = "end",
onClick,
tabIndex,
type = "button",
...props
}: MessageScrollerButtonProps) {
const { scrollToEnd, scrollToStart, stateStore } = useMessageScrollerContext();
const onClickRef = useLatest(onClick);
const subscribe = React.useCallback(
(listener: () => void) => stateStore.subscribe(listener),
[stateStore],
);
const getSnapshot = React.useCallback(() => {
const state = stateStore.getSnapshot();

return direction === "start" ? state.start : state.end;
}, [direction, stateStore]);
const isActive = React.useSyncExternalStore(subscribe, getSnapshot, getSnapshot);

const handleClick = React.useCallback(
(event: React.MouseEvent<HTMLButtonElement>) => {
if (!isActive) {
return;
}

onClickRef.current?.(event);

if (!event.defaultPrevented) {
event.currentTarget.blur();

if (direction === "start") {
scrollToStart({ behavior });
} else {
scrollToEnd({ behavior });
}
}
},
[behavior, direction, isActive, onClickRef, scrollToEnd, scrollToStart],
const { getCanScrollToEnd, scrollToEnd, subscribeCanScrollToEnd } = useMessageScrollerContext();
const isActive = React.useSyncExternalStore(
subscribeCanScrollToEnd,
getCanScrollToEnd,
getCanScrollToEnd,
);

function handleClick(event: React.MouseEvent<HTMLButtonElement>) {
if (!isActive) {
return;
}

onClick?.(event);

if (event.defaultPrevented) {
return;
}

event.currentTarget.blur();
scrollToEnd({ behavior });
}

return (
<button
{...props}
type={type}
inert={!isActive}
tabIndex={isActive ? tabIndex : -1}
data-active={isActive ? "true" : "false"}
data-direction={direction}
onClick={handleClick}
>
{children ?? <span>Scroll to {direction}</span>}
{children ?? <span>Scroll to end</span>}
</button>
);
}
Expand Down
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