here is the message log from spy++

So far those messages look innocent (different from what my trackpad sends but should still be understood by OpenMPT). We also verified that this was broken pre-1.23 when the MFC workarounds for 32-bit scrollbar positions were added.

Tried it on another touchpad and it also just worked there. Maybe it would be good to know which touchpad driver you use; is it a Synaptics driver?

as far as i can tell i'm just using microsoft default "HID-compliant touch pad" driver - can't see any manufacturer drivers anywhere in device manager - am i checking the right place?

Yeah, that sounds like it's just the default driver then, thanks. Maybe that helps with reproducing the issue, we'll see.

I just remembered this issue - I had the same problem with the touchscreen driver on my laptop a few months ago. Windows 8 / 10 translates touch events into a sequence of scroll messages that MFC is incapable of handling properly. Rather than re-implementing the whole scrolling business myself, I chose to implement touch gesture support and translate pan gestures into the desired scroll actions. Since modern touchpads typically support various touch gestures, maybe the same fix works for you.

Can you confirm if you can still reproduce the bug with the current OpenMPT version?

Works fine now :) Thank you!

Great :) So it was really caused by the touch gesture to mouse event translation layer in Windows 10 then.