fix(webui): avoid mobile welcome composer overlap

webui/src/components/thread/ThreadViewport.tsx

@@ -532,7 +532,7 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
         ref={scrollRef}
         className={cn(
           "thread-viewport-scrollbar absolute inset-0 scroll-auto scrollbar-thin",
-          hasMessages && hasVerticalOverflow ? "overflow-y-auto" : "overflow-hidden",
+          hasVerticalOverflow ? "overflow-y-auto" : "overflow-hidden",
           "[&::-webkit-scrollbar]:w-1.5",
           "[&::-webkit-scrollbar-thumb]:rounded-full",
           "[&::-webkit-scrollbar-thumb]:bg-muted-foreground/30",
@@ -574,8 +574,11 @@ export const ThreadViewport = forwardRef<ThreadViewportHandle, ThreadViewportPro
         ) : (
           <div ref={contentRef} className="mx-auto flex min-h-full w-full max-w-[72rem] flex-col px-3 sm:px-4">
             <div className="flex w-full flex-1 flex-col items-center pb-[calc(0.75rem+env(safe-area-inset-bottom))] pt-6 sm:justify-center sm:py-12">
-              <div className="relative flex w-full max-w-[58rem] flex-1 flex-col items-center justify-end gap-5 sm:block sm:flex-none">
-                <div className="pointer-events-none absolute inset-0 flex items-center justify-center sm:pointer-events-auto sm:static sm:block">
+              <div
+                data-testid="thread-welcome-layout"
+                className="relative grid w-full max-w-[58rem] flex-1 grid-rows-[minmax(min-content,1fr)_auto] gap-8 sm:block sm:flex-none"
+              >
+                <div className="flex min-h-0 items-center justify-center sm:absolute sm:inset-x-0 sm:bottom-[calc(100%+2rem)]">
                   {emptyState}
                 </div>
                 <div className="w-full">{composer}</div>