draft-js: clean up useless divs from HTML when pasting content

Summary:
When pasting rich (HTML) content into a Draft editor, the generated blocks
are flattened such that they can be represented in the internal state model.
Until a change to the tree-based model is fully implemented, that may cause
formatting to be lost if the content is wrapped in multiple levels of divs,
as the parsing does not traverse them deeply.

Improve the amount of formatting that can be preserved in the current model
by stripping out divs considered 'useless' (with no text or styling), such
that more important elements can be 'brought to the surface' and parsed
in a more pleasant manner.

Reviewed By: claudiopro

Differential Revision: D15923965

fbshipit-source-id: 72824ddba69b5d08374f505187ed4a6ea7c4d573

src/model/encoding/__tests__/__snapshots__/convertFromHTMLToContentBlocks2-test.js.snap

@@ -2967,6 +2967,71 @@ lorem ipsum
 ]
 `;
 
+exports[`eliminates useless blocks when experimentalTreeDataSupport is disabled 1`] = `false`;
+
+exports[`eliminates useless blocks when experimentalTreeDataSupport is disabled 2`] = `
+Array [
+  Object {
+    "characterList": Array [
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+    ],
+    "data": Object {},
+    "depth": 0,
+    "key": "key2",
+    "text": "Hello",
+    "type": "unstyled",
+  },
+  Object {
+    "characterList": Array [
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+      Object {
+        "entity": null,
+        "style": Array [],
+      },
+    ],
+    "data": Object {},
+    "depth": 0,
+    "key": "key3",
+    "text": "World",
+    "type": "unstyled",
+  },
+]
+`;
+
 exports[`highlighted text should be recognized and considered styled characters 1`] = `
 Array [
   Immutable.Record {

src/model/encoding/__tests__/convertFromHTMLToContentBlocks2-test.js

@@ -254,6 +254,22 @@ test('does not convert deeply nested html blocks when experimentalTreeDataSuppor
   });
 });
 
+test('eliminates useless blocks when experimentalTreeDataSupport is disabled', () => {
+  const html_string = `
+    <div>
+      <div>
+        <div>Hello</div>
+      </div>
+      <div>World</div>
+    </div>
+  `;
+
+  expect(AreTreeBlockNodesEquivalent(html_string)).toMatchSnapshot();
+  assertConvertFromHTMLToContentBlocks(html_string, {
+    experimentalTreeDataSupport: false,
+  });
+});
+
 SUPPORTED_TAGS.forEach(tag =>
   testConvertingAdjacentHtmlElementsToContentBlocks(tag, true),
 );

src/model/encoding/convertFromHTMLToContentBlocks2.js

@@ -658,6 +658,26 @@ class ContentBlocksBuilder {
     }
   }
 
+  /**
+   * Remove 'useless' container nodes from the block config hierarchy, by
+   * replacing them with their children.
+   */
+
+  _hoistContainersInBlockConfigs(
+    blockConfigs: Array<ContentBlockConfig>,
+  ): List<ContentBlockConfig> {
+    const hoisted = List(blockConfigs).flatMap(blockConfig => {
+      // Don't mess with useful blocks
+      if (blockConfig.type !== 'unstyled' || blockConfig.text !== '') {
+        return [blockConfig];
+      }
+
+      return this._hoistContainersInBlockConfigs(blockConfig.childConfigs);
+    });
+
+    return hoisted;
+  }
+
   // ***********************************************************************
   // The two methods below are used for backward compatibility when
   // experimentalTreeDataSupport is disabled.
@@ -667,9 +687,8 @@ class ContentBlocksBuilder {
    * text content.
    */
   _toFlatContentBlocks(blockConfigs: Array<ContentBlockConfig>) {
-    const l = blockConfigs.length - 1;
-    for (let i = 0; i <= l; i++) {
-      const config = blockConfigs[i];
+    const cleanConfigs = this._hoistContainersInBlockConfigs(blockConfigs);
+    cleanConfigs.forEach(config => {
       const {text, characterList} = this._extractTextFromBlockConfigs(
         config.childConfigs,
       );
@@ -680,7 +699,7 @@ class ContentBlocksBuilder {
           characterList: config.characterList.concat(characterList),
         }),
       );
-    }
+    });
   }
 
   /**