Inline filler (ZWS) is removed while typing inside empty inline element

[f1ames]

I have completely changed this issue description, see why in https://github.com/ckeditor/ckeditor5-engine/issues/1409#issuecomment-391973289 below.

1. Go to https://ckeditor5.github.io/.
2. Replace whole content with Foo Bar and place selection on the end.
3. Turn on Bold.
   • Now inline filler is inserted.
4. Type something.
   • Inline filler is removed.

Expected

Inline filler is not removed until selection is moved from text node.

Actual

Inline filler is removed while typing inside empty inline element.

There are cases when it behaves correctly. Follow the below steps (also shown in below gif) to see how it should behave:

1. Turn Bold off.
   • Now inline filler is inserted.
2. Type something.
   • Inline filler is not removed.

[f1ames]

After investigating inline fillers and rendering behaviour a little more, it turned out that actually an expected behaviour was treated as a bug in this issue. The fact that inline filler is removed while typing inside empty inline elements is an actual issue (see https://github.com/ckeditor/ckeditor5-engine/issues/1342#issuecomment-391422791 and ckeditor/ckeditor5-engine#1355 (comment)) so the opposite to what was reported 🙃 I have adjusted the issue description.