Glider, if they wanted to make it unstable or even neutraly stable, they would hve taken great care that the ailerons had the same sensitivity. It's quite unpleasant to have to make wide move in the roll axis when you've got a narrow travel range longitudinally.
The fact is that many bi-plans were marginally stable (inherent to their shape and short fuselage). Perhaps that experienced professional military pilots with years of flying the biplans in the 30's didn't bother that much that Spit annoying characteristic in regard of the general perfs improvement.
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