fix(mothership): suppress shimmer over executing tool rows and seed chase interrupt baseline

Author: TheodoreSpeaks

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

@@ -857,12 +857,14 @@ function MessageContentInner({
   // output actually arrives: hidden while the trailing text is visibly
   // revealing or any lane streamed within the quiet period, shown otherwise.
   // Immediately on an empty turn — nothing has arrived yet. A null label means
-  // a just-opened lane's delegating shimmer owns the state.
+  // a just-opened lane's delegating shimmer owns the state, and a visible
+  // executing tool row already spins — the turn-level shimmer would double it.
   const thinkingLabel = deriveThinkingLabel(blocks)
+  const hasExecutingTool = assistantMessageHasVisibleExecutingTool(blocks)
   const showShimmer =
     thinkingExpanded &&
     thinkingLabel !== null &&
-    (segments.length === 0 || (isStreamIdle && !trailingStreamActivity))
+    (segments.length === 0 || (isStreamIdle && !trailingStreamActivity && !hasExecutingTool))
 
   return (
     <div>

apps/sim/lib/core/utils/smooth-bottom-chase.ts

@@ -98,14 +98,23 @@ export function createSmoothBottomChase(
     raf = requestAnimationFrame(step)
   }
 
+  // Seed the upward-move interrupt baseline at (re)start so a user scroll-up
+  // between the kick and the first frame parks the loop immediately — without
+  // it the first step has no baseline and writes one downward frame against
+  // the user (relevant on the teardown kickUntil, where the gesture listeners
+  // are already gone).
+  const start = () => {
+    if (raf !== null) return
+    lastTop = target.getTop()
+    raf = requestAnimationFrame(step)
+  }
+
   return {
     isActive: () => raf !== null,
-    kick: () => {
-      if (raf === null) raf = requestAnimationFrame(step)
-    },
+    kick: start,
     kickUntil: (durationMs: number) => {
       deadline = Math.max(deadline, performance.now() + durationMs)
-      if (raf === null) raf = requestAnimationFrame(step)
+      start()
     },
     cancel: park,
   }