If I am editing an action that already has a variable and something else in it[1] and delete a variable then enter a % to get the "pick a variable" dropdown, name completion breaks slightly. My typing still acts as a filter. But when I hit enter to select something anything I've typed appears after the variable name.
If I start with "%OUTPUT_DIR%\Bin", delete the "%OUTPUT_DIR%" bit so I just have "\Bin" then put a % at the start I end up with "%OUTPUT_DIR%out\Bin" from typing "%out{enter}".
It works, but it's kind of annoying to have to keep deleting the extra bits. A "rename variable in the whole script" command would be useful here too.
[1] Specifically tested: file rename; copy file(s).