@J-Hilk That's interesting info, thanks. I'm using an external mouse, but the machine is indeed a Macbook with the trackpad set to clicking on a tap—so I guess it might be possible that Qt processes some low-level events in a weird way.
However, while I tried changing the system settings it turned out my MacOS forgot that it has a trackpad at all, and doesn't show those options even though input works fine. I don't think this new problem of mine is related to dragging-on-click since dragging-on-click is older IIRC (and occurs on Linux too, judging from the link)—but apparently I will now be fighting MacOS instead for a while, as quick googling didn't uncover anything on this topic. I also thought that I already checked if dragging-on-click persists with all input-tuning apps turned off, but perhaps I misremembered. Will report after I wrangle the OS and make it behave properly.