Which one is more correct ?
A) Remain on the chair until lift has stopped.
B) Remain on the chair until lift is stopped.
Also,
A) Do not attempt to leave the lift until the landing gate is opened fully.
B) Do not attempt to leave the lift until the landing gate has opened fully.
Your first sentence is missing a the in front of lift.
Assuming that is in, the the one that is more correct is A because both tenses match (has and stopped):
Remain on the chair until the lift has stopped.
Although, I would probably rewrite this sentence as:
Please remain seated until the lift has stopped.
For the 2nd sentence, B) is correct, but A can be rewritten to say:
A) Do not attempt to leave the lift until the landing gate is fully open.
Like the above, both of the tenses need to match for it to be correct (although in the 1st set of sentences, is and stop wouldn't make sense because stop is not an adjective unlike stopped).
I do prefer the "fully" to precede the word "open" in both cases.
