When multiple monitors are connected, shaking vigorously becomes a futile effort because the amount of movement required in such a short time means it's impossible to do without accidentally moving the mouse over to a different monitor.
Likewise the black bars that cover the rest of the window when resized to a different aspect ratio than the game render (such as when maximized/fullscreen on an ultrawide monitor) also interrupt the input.
Block areas being unresponsive is probably more of a technical problem in the engine rather than something necessarily dependent on my setup. But it was consistently registering only about 1 in every 3 clicks, even with the game running at less than 10% CPU and 5% GPU.
It also rather frequently seemed like Katharine would attack in the middle of the parry animation, but that was more of an annoyance than a gamebreaking issue.