fix(rich-text-input): placeholder bug (#1099)

* fix(rich-text-input): placeholder bug

Author: montezume

src/components/internals/rich-text-plugins/rich-text-plugins.js

@@ -18,12 +18,19 @@ const plugins = [
           editor.value.document.nodes.map(node => node.text).toArray()
             .length === 1;
         const blocks = editor.value.blocks.map(block => block.type).toArray();
+
         const hasOneBlock = blocks.length === 1;
         const isParagraph = blocks[0] && blocks[0] === BLOCK_TAGS.p;
 
         const shouldUsePlaceholder =
-          isEditorEmpty && hasOneNode && hasOneBlock && isParagraph;
+          !editor.hasPlaceholder &&
+          isEditorEmpty &&
+          hasOneNode &&
+          hasOneBlock &&
+          isParagraph;
 
+        // eslint-disable-next-line no-param-reassign
+        editor.hasPlaceholder = shouldUsePlaceholder;
         return shouldUsePlaceholder;
       },
     },