Environment

Windows build number: Microsoft Windows [版本 10.0.18362.836]
Windows Terminal version (if applicable): 1.0.1401.0

Steps to reproduce

1. type some text like "abcde"
2. move the cursor to the beginning
3. switch ime to Microsoft Pinyin
4. type some text like "xxx"
5. see the screenshot

Expected behavior

Like cmd.exe or other input control, draw a background for the composition text.

Actual behavior