Currently there is a very annoying bug with ; style comments — autocompletion doesn't turn itself off because it doesn't recognize the last caret position as a comment.line.ahk scope
(so, it's only turned off when you edit comments since then you move away from the end of the line)

changing
<string>(^\s*|\s+)(;)(.*)$</string>
to
<string>(^\s*|\s+)(;)(.*)(\n?)$</string>
fixes it for me, so I'll submit a patch, though not 100% certain this is the proper fix