You should be able to tab out of a normal long text field, as long as it’s not the last field on your grid. For some reason, Airtable doesn’t let us tab out of the last field.
If your long text field is a rich text field, option-tab on the Mac will tab you out of the field. I don’t know if the same shortcut equivalent will apply on Windows (alt-tab is the equivalent on Windows), because I think you guys use alt-tab to cycle through applications?? Perhaps there is some other keyboard combo with the tab key that you can find on Windows that gets you out of the rich text field.
Although the same caveat applies if it’s the last field on the grid — Airtable won’t let us tab out of the last field.