fix(mothership): debt-aware drain fast-path and settle-window handle retention

Author: TheodoreSpeaks

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

@@ -352,16 +352,19 @@ export function MothershipChat({
           floorDrainRafRef.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) {
+        // Instant-clear only when the whole remaining debt sits BELOW the
+        // viewport (debt ≤ distance-from-bottom) — then the shrink is
+        // invisible. A merely-unpinned viewport with debt larger than its
+        // slack would still clamp, so it keeps the eased drain instead.
+        const distance = el.scrollHeight - el.scrollTop - el.clientHeight
+        const debt = current - target
+        if (debt <= 1 || debt <= distance) {
           sizerFloorAppliedRef.current = 0
           floorDrainRafRef.current = 0
           sizer.style.minHeight = ''
           return
         }
+        const next = Math.floor(current - Math.max(1, debt * SMOOTH_CHASE_RATE))
         sizerFloorAppliedRef.current = next
         sizer.style.minHeight = `${next}px`
         floorDrainRafRef.current = requestAnimationFrame(drain)

apps/sim/hooks/use-auto-scroll.ts

@@ -66,10 +66,21 @@ export function useAutoScroll(isStreaming: boolean) {
     containerRef.current = el
   }, [])
 
+  /**
+   * Cancels the previous teardown's settle window (chase, temp listeners,
+   * removal timer). Invoked when a new stream starts — a stop-then-resend must
+   * not leave the old settle chase writing beside the new stream's chase — and
+   * on unmount.
+   */
+  const settleCleanupRef = useRef<(() => void) | null>(null)
+  useEffect(() => () => settleCleanupRef.current?.(), [])
+
   useEffect(() => {
     if (!isStreaming) return
     const el = containerRef.current
     if (!el) return
+    settleCleanupRef.current?.()
+    settleCleanupRef.current = null
 
     /**
      * Eased bottom-chase shared by the mutation observer and the seed below —
@@ -224,10 +235,19 @@ export function useAutoScroll(isStreaming: boolean) {
       }
       el.addEventListener('wheel', cancelOnGesture, { passive: true })
       el.addEventListener('touchmove', cancelOnGesture, { passive: true })
-      setTimeout(() => {
+      const removeGestureGuard = () => {
         el.removeEventListener('wheel', cancelOnGesture)
         el.removeEventListener('touchmove', cancelOnGesture)
+      }
+      const guardTimeout = setTimeout(() => {
+        removeGestureGuard()
+        settleCleanupRef.current = null
       }, POST_STOP_SETTLE_WINDOW + 100)
+      settleCleanupRef.current = () => {
+        chase.cancel()
+        clearTimeout(guardTimeout)
+        removeGestureGuard()
+      }
     }
   }, [isStreaming])