To reproduce, run the mouse demo, click and drag to start drawing a box, then scroll up or down.
Also, in the micro editor you can't start a selection with click-drag, them scroll with the mouse wheel (you can in vim).

The problem is here:

The wasbtn check seems to be there to work around some terminal bug, but I don't know why. If I remove the check and always report a wheel event the mouse demo and scrolling in micro work as expected.

This can also result in wheel events not during a click-drag being reported as button presses if the tcell application spawns another application on a click event. The child app gets the mouse release event and wasbtn never gets reset.