getLeaf() should ignore non-leaf blots

[luin]

Previously, Quill#getLeaf() can return non-leaf blots, which is unexpected. For example, it breaks Selection#getBounds() as it calls leaf.position(offset, true), which is only provided by leaf blots.

The reason Quill#getLeaf() may return a non-leaf blot is, in ParentBlot#path(), if this.children.find(index, inclusive) return [null, 0] (aka the parent blot is empty), the last element of the path would be the parent blot itself, causing Scroll#leaf() gets the parent blot.

The reason that a parent blot can be empty is in Container#optimize(), we don't call container's children's optimize() function, so the children who are parent blots don't have a chance to create a default child. I haven't reproduced this in practice so not 100% sure, but programmatically a reproducible example can be found in the added test case in this PR.

---

This PR ensures Scroll#leaf() always returns a leaf or null. In fact, this won't happen if we get Container#optimize() fixed, so it may be debatable whether we should do the check. However, IMO people may override Container#optmize() and forget optimizing children even we fix it on our side, so to make Scroll#leaf() complete, a check would be helpful.

[rockwotj]

this.quill is undefined, so this test fails for our fork here.

[rockwotj]

The following patch fixes the test here:

diff --git a/test/unit/core/selection.js b/test/unit/core/selection.js
index 164f5f8b..ef1d1832 100644
--- a/test/unit/core/selection.js
+++ b/test/unit/core/selection.js
@@ -1,6 +1,9 @@
+import Delta from 'quill-delta';
+
 import Selection, { Range } from '../../../core/selection';
 import Cursor from '../../../blots/cursor';
 import Emitter from '../../../core/emitter';
+import Editor from '../../../core/editor';
 
 describe('Selection', function() {
   beforeEach(function() {
@@ -632,12 +635,13 @@ describe('Selection', function() {
     });
 
     it('empty container', function() {
-      const selection = this.initialize(
-        Selection,
+      const [editor, selection] = this.initialize(
+        [Editor, Selection],
         '<table><tr><td data-row="a">a</td></tr></table>',
       );
-      this.quill.updateContents([{ retain: 1 }, { insert: '\n' }]);
+      editor.applyDelta(new Delta([{ retain: 1 }, { insert: '\n' }]));
       expect(selection.getBounds(2, 0)).toEqual(null);
+      this.bounds = selection.getBounds(0, 1);
     });
   });
 });