fix(mothership): single stopped tail region and drain cleanup

Author: TheodoreSpeaks

apps/sim/app/workspace/[workspaceId]/home/components/message-content/message-content.tsx

@@ -983,17 +983,21 @@ function MessageContentInner({
           </div>
         </div>
       ) : (
-        // Stopped row and actions take the slot's place in the SAME render and
-        // at the SAME extent (TAIL_REGION_CLASSES), so the swap is height-
-        // neutral by construction — no reflow for the pinned scroller to absorb.
+        // The settled tail takes the slot's place in the SAME render and at the
+        // SAME extent (TAIL_REGION_CLASSES), so the swap is height-neutral by
+        // construction — no reflow for the pinned scroller to absorb. A stopped
+        // turn shares the ONE region (status left, actions right); stacking two
+        // regions would double the tail and break neutrality.
         <>
-          {lastSegment?.type === 'stopped' && (
+          {lastSegment?.type === 'stopped' ? (
             <div className={cn(TAIL_REGION_CLASSES, 'gap-[8px]')}>
               <CircleStop className='size-[16px] flex-shrink-0 text-[var(--text-icon)]' />
               <span className='text-[14px] text-[var(--text-body)]'>Stopped by user</span>
+              {actions && <div className='ml-auto'>{actions}</div>}
             </div>
+          ) : (
+            actions && <div className={TAIL_REGION_CLASSES}>{actions}</div>
           )}
-          {actions && <div className={TAIL_REGION_CLASSES}>{actions}</div>}
         </>
       )}
     </div>

apps/sim/app/workspace/[workspaceId]/home/components/mothership-chat/mothership-chat.tsx

@@ -306,6 +306,7 @@ export function MothershipChat({
   const scrollerPaddingRef = useRef<{ top: number; bottom: number } | null>(null)
   const sizerFloorAppliedRef = useRef(0)
   const floorDrainRafRef = useRef(0)
+  useEffect(() => () => cancelAnimationFrame(floorDrainRafRef.current), [])
 
   /**
    * Sizer floor while streaming: `scrollHeight` must never dip below the
@@ -340,12 +341,14 @@ export function MothershipChat({
     if (!floorActive) {
       if (sizerFloorAppliedRef.current === 0) return
       scrollerPaddingRef.current = null
-      const pinned = el.scrollHeight - el.scrollTop - el.clientHeight <= 2
       cancelAnimationFrame(floorDrainRafRef.current)
       const drain = () => {
         const target = virtualizer.getTotalSize()
         const current = sizerFloorAppliedRef.current
         if (current === 0) return
+        // Re-checked per frame: a user scrolling away mid-drain makes the
+        // remaining shrink invisible, so finish instantly instead of clamping.
+        const pinned = el.scrollHeight - el.scrollTop - el.clientHeight <= 2
         const next = Math.floor(current - Math.max(1, (current - target) * SMOOTH_CHASE_RATE))
         if (!pinned || next - target <= 1) {
           sizerFloorAppliedRef.current = 0