Avoid extra new line in Firefox when block ends with soft new line

src/component/handlers/edit/editOnBeforeInput.js

@@ -22,6 +22,7 @@ const editOnInput = require('editOnInput');
 const getEntityKeyForSelection = require('getEntityKeyForSelection');
 const isEventHandled = require('isEventHandled');
 const isSelectionAtLeafStart = require('isSelectionAtLeafStart');
+const isSelectionAtBlockEndWithNewLine = require('isSelectionAtBlockEndWithNewLine');
 const nullthrows = require('nullthrows');
 const setImmediate = require('setImmediate');
 
@@ -162,6 +163,16 @@ function editOnBeforeInput(
       editor._latestCommittedEditorState,
     );
   }
+
+  if (!mustPreventNative) {
+    // When there is \n at the end of the block, there is one extra \n added in DraftEditorLeaf for the content to render properly in browsers
+    // editOnInput takes care of it, but we must prevent the new char to land natively between the two
+    // as in that case the check in editOnInput wouldn't detect the situation properly
+    mustPreventNative = isSelectionAtBlockEndWithNewLine(
+      editor._latestCommittedEditorState,
+    );
+  }
+
   if (!mustPreventNative) {
     // Let's say we have a decorator that highlights hashtags. In many cases
     // we need to prevent native behavior and rerender ourselves --

src/component/selection/isSelectionAtBlockEndWithNewLine.js

@@ -0,0 +1,25 @@
+/**
+ * Copyright (c) Facebook, Inc. and its affiliates.
+ *
+ * This source code is licensed under the MIT license found in the
+ * LICENSE file in the root directory of this source tree.
+ *
+ * @emails oncall+draft_js
+ * @flow strict-local
+ * @format
+ */
+'use strict';
+
+import type EditorState from 'EditorState';
+
+function isSelectionAtBlockEndWithNewLine(editorState: EditorState): boolean {
+  const selection = editorState.getSelection();
+  const anchorKey = selection.getAnchorKey();
+  const offset = selection.getStartOffset();
+
+  const block = editorState.getCurrentContent().getBlockForKey(anchorKey);
+
+  return block.getLength() === offset && block.getText()[offset - 1] === '\n';
+}
+
+module.exports = isSelectionAtBlockEndWithNewLine;