fix(mothership): single drain cadence and a settle-window gesture kill switch

Author: TheodoreSpeaks

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

@@ -340,29 +340,37 @@ export function MothershipChat({
     if (!sizer || !el) return
     if (!floorActive) {
       if (sizerFloorAppliedRef.current === 0) return
+      // A drain already in flight keeps its own rAF cadence — settle-burst
+      // commits re-enter this branch and must not add extra steps in layout,
+      // which would accelerate the release past the eased rate.
+      if (floorDrainRafRef.current !== 0) return
       scrollerPaddingRef.current = null
-      cancelAnimationFrame(floorDrainRafRef.current)
       const drain = () => {
         const target = virtualizer.getTotalSize()
         const current = sizerFloorAppliedRef.current
-        if (current === 0) return
+        if (current === 0) {
+          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) {
           sizerFloorAppliedRef.current = 0
+          floorDrainRafRef.current = 0
           sizer.style.minHeight = ''
           return
         }
         sizerFloorAppliedRef.current = next
         sizer.style.minHeight = `${next}px`
         floorDrainRafRef.current = requestAnimationFrame(drain)
       }
-      drain()
+      floorDrainRafRef.current = requestAnimationFrame(drain)
       return
     }
     cancelAnimationFrame(floorDrainRafRef.current)
+    floorDrainRafRef.current = 0
     if (!scrollerPaddingRef.current) {
       const style = getComputedStyle(el)
       scrollerPaddingRef.current = {

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

@@ -214,6 +214,20 @@ export function useAutoScroll(isStreaming: boolean) {
       // behind the input; a user who scrolled away stays put via the sticky
       // check.
       chase.kickUntil(POST_STOP_SETTLE_WINDOW)
+      // The main gesture listeners are gone, so give the settle window its own
+      // kill switch: the chase's clamp-aware interrupt catches wheel-scale
+      // moves, but a slow trackpad glide during a concurrent shrink could slip
+      // under it for up to the window's duration.
+      const cancelOnGesture = (event: Event) => {
+        if (event.type === 'wheel' && (event as WheelEvent).deltaY >= 0) return
+        chase.cancel()
+      }
+      el.addEventListener('wheel', cancelOnGesture, { passive: true })
+      el.addEventListener('touchmove', cancelOnGesture, { passive: true })
+      setTimeout(() => {
+        el.removeEventListener('wheel', cancelOnGesture)
+        el.removeEventListener('touchmove', cancelOnGesture)
+      }, POST_STOP_SETTLE_WINDOW + 100)
     }
   }, [isStreaming])