(In a document): Top Menu > Gear > Status bar > Settings > Reclaim bar height from bottom margin
That'll ensure a 0 bottom margin will actually be 0.
Whether you do get a final line of text in here is subject to whether CRe decides it has enough space to do so or not given the current font settings
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I don't *think* it actually supports negative margins, in which case what you just did was just a roundabout way of negating the status bar height, but I very well might be wrong
(ping @poire-z ^^).