That was enough of a change to fix it for me, much obliged. I suspect TheMather1 was correct with their guess that multi-monitor setups may factor into the issues. Previously, disabling 2 of my 3 allowed me to get 2 parries successfully, but the lightly-shaded click boxes didn't consistently appear before Katherine's attacks landed, so there was no way to activate the block function. With the extra time and reduced movement required, I only missed one block, and was able to pass the fight!