feat(core): add missing styles warning

packages/core/src/composables/useStylesLoadedWarning.ts

@@ -0,0 +1,17 @@
+import { onMounted } from 'vue'
+import { ErrorCode, VueFlowError, isDev } from '../utils'
+import { useVueFlow } from './useVueFlow'
+
+export function useStylesLoadedWarning() {
+  const { emits } = useVueFlow()
+
+  onMounted(() => {
+    if (isDev()) {
+      const pane = document.querySelector('.vue-flow__pane')
+
+      if (pane && !(window.getComputedStyle(pane).zIndex === '1')) {
+        emits.error(new VueFlowError(ErrorCode.MISSING_STYLES))
+      }
+    }
+  })
+}