fix(ui): stop forced auto-follow during streaming (#309)

# PR Draft: Fix sticky auto-scroll during streaming chat responses

Fixes #308

## Summary

This change makes chat auto-scroll easier to escape while assistant
output is still streaming.

The goal is to stop the viewport from repeatedly pulling the user back
toward the bottom once they begin scrolling upward to inspect earlier
content.

## Why

Before this change, streaming updates could keep reasserting
bottom-follow behavior during active rendering. That made auto-scroll
feel sticky and forced users to scroll repeatedly or forcefully just to
review earlier parts of an in-progress response.

The intended behavior is simpler: once the user scrolls upward to leave
follow mode, the UI should respect that decision instead of fighting it
during subsequent stream updates.

## What Changed

1. Removed render-time force-bottom behavior from the shared
follow-scroll helper path.
2. Updated streamed reasoning output to restore scroll without forcing
the viewport back to the bottom.
3. Updated streamed tool-call output to use the same non-forcing restore
behavior.

## Scope Boundaries

Included:

- Sticky auto-scroll behavior during streamed chat output
- Shared follow-scroll behavior used by streamed nested panes
- Reasoning and tool-call streaming paths that reused the same forced
follow behavior

Not included:

- A full rewrite of the virtualized message list follow model
- Broader scroll UX changes outside the streaming follow/escape behavior
- Unrelated UI or plugin configuration changes in the worktree

## Technical Notes

The core problem was not basic auto-scroll itself, but a render-time
path that could keep forcing bottom-follow behavior while new streamed
content was arriving.

That meant a user's attempt to scroll upward could be overridden
repeatedly by subsequent stream updates, which is why the auto-scroll
felt sticky. The fix removes that override and keeps render-time
restoration dependent on the current follow state instead.

## Files Changed

- `packages/ui/src/lib/follow-scroll.tsx`
- `packages/ui/src/components/message-block.tsx`
- `packages/ui/src/components/tool-call.tsx`

## Verification

Performed:

1. Reproduced the sticky auto-scroll behavior with a long multi-line
streaming response.
2. Verified that scrolling upward during streaming now disengages follow
more naturally in the affected streamed panes.
3. Ran `npm run typecheck --workspace @codenomad/ui`.
4. Ran `npm run build --workspace @codenomad/ui`.

Build note:

- The UI typecheck passes.
- The UI build succeeds.
- The build still emits existing third-party and chunk-size warnings
unrelated to this change.

## Risks and Follow-up

1. The broader scroll-follow model is still more heuristic-heavy than
ideal, so there may be future follow-up work to simplify it further.
2. This PR intentionally applies the smallest targeted fix to the known
snap-back path instead of rewriting the full chat scroll system.

---------

Co-authored-by: Shantur Rathore <i@shantur.com>

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

@@ -1344,9 +1344,7 @@ function ReasoningStreamOutput(props: {
     if (preRef && preRef.textContent !== nextText) {
       preRef.textContent = nextText
     }
-    if (followScroll.autoScroll()) {
-      followScroll.restoreAfterRender({ forceBottom: true })
-    }
+    followScroll.restoreAfterRender()
     notifyContentRendered()
   })
 

packages/ui/src/components/tool-call.tsx

@@ -454,7 +454,7 @@ function ToolCallDetails(props: {
 
   createEffect(() => {
     if (followScroll.autoScroll()) {
-      scrollHelpers.restoreAfterRender({ forceBottom: true })
+      scrollHelpers.restoreAfterRender()
     }
   })
 

packages/ui/src/components/tool-call/types.ts

@@ -47,7 +47,7 @@ export interface ToolScrollHelpers {
   registerContainer(element: HTMLDivElement | null, options?: { disableTracking?: boolean }): void
   handleScroll(event: Event & { currentTarget: HTMLDivElement }): void
   renderSentinel(options?: { disableTracking?: boolean }): JSXElement | null
-  restoreAfterRender(options?: { forceBottom?: boolean }): void
+  restoreAfterRender(): void
 }
 
 export interface ToolRendererContext {

packages/ui/src/lib/follow-scroll.tsx

@@ -16,7 +16,7 @@ export interface FollowScrollHelpers {
   registerContainer: (element: HTMLDivElement | null | undefined, options?: { disableTracking?: boolean }) => void
   handleScroll: (event: Event & { currentTarget: HTMLDivElement }) => void
   renderSentinel: (options?: { disableTracking?: boolean }) => JSXElement | null
-  restoreAfterRender: (options?: { forceBottom?: boolean }) => void
+  restoreAfterRender: () => void
   autoScroll: Accessor<boolean>
 }
 
@@ -183,7 +183,7 @@ export function createFollowScroll(options: FollowScrollOptions): FollowScrollHe
     return <div ref={setBottomSentinel} aria-hidden="true" class={options.sentinelClassName} style={{ height: "1px" }} />
   }
 
-  const restoreAfterRender = (config?: { forceBottom?: boolean }) => {
+  const restoreAfterRender = () => {
     const container = scrollContainerRef
     if (container && hasUserScrollIntent() && !isAtBottom(container)) {
       if (autoScroll()) {
@@ -195,7 +195,10 @@ export function createFollowScroll(options: FollowScrollOptions): FollowScrollHe
       return
     }
 
-    const shouldFollow = config?.forceBottom ?? autoScroll()
+    // Never let a render-time caller force follow mode back on after the user
+    // has already escaped it. Staying pinned should depend on the current
+    // follow state, not on a caller opting into forceBottom.
+    const shouldFollow = autoScroll()
     requestAnimationFrame(() => {
       restoreScrollPosition(shouldFollow)
       if (shouldFollow) {