feat: Transitions, spacing, scroll fade, prompt area update (#11168)

Co-authored-by: Github Action <action@github.com>
Co-authored-by: Copilot <198982749+Copilot@users.noreply.github.com>
Co-authored-by: aaroniker <4730431+aaroniker@users.noreply.github.com>

packages/ui/src/components/message-part.tsx

@@ -49,6 +49,7 @@ import { Tooltip } from "./tooltip"
 import { IconButton } from "./icon-button"
 import { createAutoScroll } from "../hooks"
 import { createResizeObserver } from "@solid-primitives/resize-observer"
+import { MorphChevron } from "./morph-chevron"
 
 interface Diagnostic {
   range: {
@@ -415,7 +416,7 @@ export function UserMessageDisplay(props: { message: UserMessage; parts: PartTyp
               toggleExpanded()
             }}
           >
-            <Icon name="chevron-down" size="small" />
+            <MorphChevron expanded={expanded()} />
           </button>
           <div data-slot="user-message-copy-wrapper">
             <Tooltip