After upgrading to High Sierra (10.13), Airmail v3.5 seems to display issues when in full screen mode.

When composing an email, the To/CC fields would usually autocomplete/suggest contacts as you start typing their name/address. However, in High Sierra full screen mode, this functionality is not present. If you go to windowed mode, it behaves as expected.

Also, spelling autocorrect also seems to have display issues in full screen mode.