Here is the instruction to reproduce this bug:

1. Start a normal CMD.exe session
2. set PROMPT=$P$G^G (note: ^G = 1 character, type Ctrl+G)
3. Inject a clink session
4. Type a line that is longer than window width so that it wraps to the second line; just type it, do not hit Enter
5. Hit ↑
6. The first character is left uncleared, and also cursor position is weird (try ←)

Tested on clink 0.4.8 and Windows 10 Creators Update.