Composition characters behave unexpected during composing when cursor is on trix-cursor-target span

##### Steps to Reproduce

The demo below is performed on https://trix-editor.org/

https://user-images.githubusercontent.com/15135426/114520557-28d32e00-9c74-11eb-8f1e-1a5dc7224367.mov

1. Upload an attachment(image).
2. Start typing characters that need composition after the attachment.
3. If it is composing, the characters will align vertically rather than horizontally, which is unexpected.
4. Press enter to end the composition, the characters align horizontally as expected but with a leading redundant `Ｔ` which is used to compose the first character `現`.

Looks like the strange composition behavior will happen when cursor is on `` or `` .

Thanks for go through this：）

##### Details

* Trix version: 1.3.1
* Browser name and version: Chrome(89.0.4389.90)
* Operating system: MacOS 10.13.4