Sorry...It may not the bug of libinput, i've downgrade my libinput to the version 1.8.2-1, but the bug still exists... Probably the problem has caughted by KDE, so just close this bug tracking for sure ...
- Bug#878744: All input method failed on Qt-based... Anthony Lee
- Bug#878744: close request :( Anthony Lee
- Bug#878744: marked as done (All input meth... Debian Bug Tracking System