I think the control rework caused mouse sensitivity to treat horizontal and vertical movement at a 1:1 ratio, which means that when horizontal is "conveniently fast", vertical becomes extremely twitchy, the smallest movement sending it to 90 deg up/down.
Previously it felt just right, so trying to reproduce a similar ratio or behaviour should work better.
On a related note, mouse look/control as a whole feels a lot different than before, but I can't quite tell in what way. Maybe as if turning was done in small but fixed amounts and input specifies how many of such discrete jumps are performed...? There also appears to be a loss of precision for small movements, while large/fast movements are capped at a certain rate, so essentailly working the opposite of how it should: slow movement should be smoother and fast should be more coarse.
Actually, I think it's mouse smoothing/dampening working in reverse. Small movements are amplified and large movements are attenuated. The faster the mouse moves across the same distance, the smaller the resulting turn rate becomes.
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