Missing New button on One2Many with more than 2 records
From https://discuss.tryton.org/t/one2many-new-button-disappear-after-adding-records/6089 The field size is wrongly computed as it deduct an Array from size for which JavaScript as weird behavior: ```javascript >> -1 - [] -1 >> -1 - [1] -2 >> -1 - [1, 2] NaN ```
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