When scrolling through the hotbar with the mouse I get a skipped slot every three slots like

so: 1-2-4-5-7-8...

According to MC-123773 this should be fixed with "Discrete Scrolling" enabled, but that option doesn't change anything.

Playing with the sensitivity leads to different results but nether one where one "click" of the wheel matches one move on the hotbar.

evtest reports the following consistent for each "click"::

Event: time 1568458547.186289, type 2 (EV_REL), code 8 (REL_WHEEL), value -1
Event: time 1568458547.186289, type 2 (EV_REL), code 11 , value -120

Event: time 1568458546.668323, type 2 (EV_REL), code 8 (REL_WHEEL), value 1
Event: time 1568458546.668323, type 2 (EV_REL), code 11 , value 120