fix(Toaster): smoother visibility transition for stacked toasts (#4367)

Author: sifferhans

src/theme/toaster.ts

@@ -1,7 +1,7 @@
 export default {
   slots: {
     viewport: 'fixed flex flex-col w-[calc(100%-2rem)] sm:w-96 z-[100] data-[expanded=true]:h-(--height) focus:outline-none',
-    base: 'pointer-events-auto absolute inset-x-0 z-(--index) transform-(--transform) data-[expanded=false]:data-[front=false]:h-(--front-height) data-[expanded=false]:data-[front=false]:*:invisible data-[state=closed]:animate-[toast-closed_200ms_ease-in-out] data-[state=closed]:data-[expanded=false]:data-[front=false]:animate-[toast-collapsed-closed_200ms_ease-in-out] data-[swipe=move]:transition-none transition-[transform,translate,height] duration-200 ease-out'
+    base: 'pointer-events-auto absolute inset-x-0 z-(--index) transform-(--transform) data-[expanded=false]:data-[front=false]:h-(--front-height) data-[expanded=false]:data-[front=false]:*:opacity-0 data-[front=false]:*:transition-opacity data-[front=false]:*:duration-100 data-[state=closed]:animate-[toast-closed_200ms_ease-in-out] data-[state=closed]:data-[expanded=false]:data-[front=false]:animate-[toast-collapsed-closed_200ms_ease-in-out] data-[swipe=move]:transition-none transition-[transform,translate,height] duration-200 ease-out'
   },
   variants: {
     position: {