IntelliJ is showing false positives on unused elements even though one can position the carrot on a unused element, press Ctrl+B, and find the location where the unused element is being used.

Correct me if I am wrong. ListItems doesn't looks like unused. AddItem and deleteItem vals look like unused assignments

Refer to the next thread.

If you are referring the extension function. It is probably that method doesn't use any fields or methods from the extended class

Thank you, created https://youtrack.jetbrains.com/issue/KT-39454. This is not reproduced in 1.4-M2 IDE plugin.

Good to know. So this should mean that the issue won't be present by the time the stable version of the Kotlin 1.4 plugin arrives.