Exclude trailing block for type toggling if at zero offset

Resolves facebookarchive#86.

In cases where the user triple-clicks on a block, then toggles the block type, it feels weird to have the trailing block also change block type.

The `SelectionState` resulting from the triple-click is correct, but to improve the interaction here, let's discard the trailing block for this modifier.

Test Plan:

View rich.html, add three lines of text.

- Triple-click first line, toggle H1. Verify that only the first line toggles.
- Repeat with second line, verify that only the second line toggles.
- Repeat with third line (no trailing block), verify that only the third line toggles.

Also select across multiple blocks by dragging, verify that toggling block types properly affects all visibly selected blocks.

src/model/modifier/RichTextEditorUtil.js

@@ -18,6 +18,7 @@ var DraftModifier = require('DraftModifier');
 var EditorState = require('EditorState');
 
 var adjustBlockDepthForContentState = require('adjustBlockDepthForContentState');
+var nullthrows = require('nullthrows');
 
 import type ContentState from 'ContentState';
 import type {DraftBlockType} from 'DraftBlockType';
@@ -290,6 +291,23 @@ var RichTextEditorUtil = {
     var startKey = selection.getStartKey();
     var endKey = selection.getEndKey();
     var content = editorState.getCurrentContent();
+    var target = selection;
+
+    // Triple-click can lead to a selection that includes offset 0 of the
+    // following block. The `SelectionState` for this case is accurate, but
+    // we should avoid toggling block type for the trailing block because it
+    // is a confusing interaction.
+    if (startKey !== endKey && selection.getEndOffset() === 0) {
+      var blockBefore = nullthrows(content.getBlockBefore(endKey));
+      endKey = blockBefore.getKey();
+      target = target.merge({
+        anchorKey: startKey,
+        anchorOffset: selection.getStartOffset(),
+        focusKey: endKey,
+        focusOffset: blockBefore.getLength(),
+        isBackward: false,
+      });
+    }
 
     var hasMedia = content.getBlockMap()
       .skipWhile((_, k) => k !== startKey)
@@ -306,7 +324,7 @@ var RichTextEditorUtil = {
 
     return EditorState.push(
       editorState,
-      DraftModifier.setBlockType(content, selection, typeToSet),
+      DraftModifier.setBlockType(content, target, typeToSet),
       'change-block-type'
     );
   },