feat(settings): add streaming auto-follow toggle

New Streaming section on the Chat settings page with a checkbox that
controls whether the viewport follows new content while a response
streams. Default stays on. With it off, the anchored user message still
parks at the top on send, but no glide or end-follow correction runs and
the list's maintain-scroll-at-end stays disabled; the scroll-to-bottom
pill and session open keep scrolling explicitly.

Persisted through desktop settings like the other chat toggles (auto-save
diff, authoritative apply, sanitize), registered in settings search, and
localized in every locale.

packages/ui/src/components/chat/MessageList.tsx

@@ -1021,6 +1021,10 @@ const TimelineList = React.memo(({
     rowContext,
 }: TimelineListProps) => {
     const listRef = React.useRef<LegendListRef | null>(null);
+    // With streaming auto-follow off, content growth must never move the
+    // viewport; explicit commands (the scroll-to-bottom pill, session open)
+    // still scroll through the imperative handle.
+    const streamingAutoFollowEnabled = useUIStore((state) => state.streamingAutoFollowEnabled);
     const isAtEndRef = React.useRef(true);
 
     const setListRef = React.useCallback((list: LegendListRef | null) => {
@@ -1067,7 +1071,7 @@ const TimelineList = React.memo(({
                 contentInsetEndAdjustment={composerOverlayHeight}
                 // While a turn is anchored, the reserved end space — not the
                 // live edge — defines where the viewport rests.
-                maintainScrollAtEnd={anchoredEndSpace
+                maintainScrollAtEnd={anchoredEndSpace || !streamingAutoFollowEnabled
                     ? false
                     : { animated: false, on: { dataChange: true, itemLayout: true, layout: true } }}
                 // Prepending older history must not move what the user is

packages/ui/src/components/sections/openchamber/OpenChamberVisualSettings.tsx

@@ -302,6 +302,8 @@ export const OpenChamberVisualSettings: React.FC<OpenChamberVisualSettingsProps>
     const sessionGoalDefaultBudget = useUIStore(state => state.sessionGoalDefaultBudget);
     const setSessionGoalDefaultBudget = useUIStore(state => state.setSessionGoalDefaultBudget);
     const setShowReasoningTraces = useUIStore(state => state.setShowReasoningTraces);
+    const streamingAutoFollowEnabled = useUIStore(state => state.streamingAutoFollowEnabled);
+    const setStreamingAutoFollowEnabled = useUIStore(state => state.setStreamingAutoFollowEnabled);
     const collapsibleThinkingBlocks = useUIStore(state => state.collapsibleThinkingBlocks);
     const setCollapsibleThinkingBlocks = useUIStore(state => state.setCollapsibleThinkingBlocks);
 
@@ -1839,6 +1841,20 @@ export const OpenChamberVisualSettings: React.FC<OpenChamberVisualSettingsProps>
                                         )}
                                     </SettingsSection>
                                 )}
+                                <SettingsSection
+                                    title={t('settings.openchamber.visual.section.streaming')}
+                                    settingsItem="chat.streaming"
+                                    contentClassName={SETTINGS_OPTION_STACK_CLASS}
+                                >
+                                    <SettingsCheckboxRow
+                                        checked={streamingAutoFollowEnabled}
+                                        onChange={setStreamingAutoFollowEnabled}
+                                        label={t('settings.openchamber.visual.field.streamingAutoFollow')}
+                                        ariaLabel={t('settings.openchamber.visual.field.streamingAutoFollowAria')}
+                                        info={t('settings.openchamber.visual.field.streamingAutoFollowInfo')}
+                                        settingsItem="chat.streaming-auto-follow"
+                                    />
+                                </SettingsSection>
 
                                 {(shouldShow('collapsibleUserMessages') || shouldShow('stickyUserHeader') || (shouldShow('promptNavigatorEnabled') && !isVSCode) || shouldShow('wideChatLayout') || shouldShow('splitAssistantMessageActions') || shouldShow('codeBlockLineWrap')) && (
                                 <SettingsSection