Does F-35 flight stick have a trackball?
That appears to be a button, not a trackball.
From pg. 17 in a presentation, Active Stick &amp; Throttle for F-35 - Joseph Krumenacker - NAVAIR Flight Controls / JSF Vehicle Systems - 16 October 2008, found here:
It is used to engage/disengage stick sensitivity in hover. Inadvertent movement of the stick would disengage the altitude hold.
Engaging that S-5 button increases the force to needed to 'disengage'.


solution was a pilot-engageable “gripper” mode, in
which an altitude-hold augmentation could be
quickly engaged and disengaged
– with gripper engaged, longitudinal stick breakout
force is significantly increased to prevent
inadvertent disengagement
Pros

Hover stick force is “significantly higher” and provides pilot direct tactile feedback and
confirmation that the flight control system is in control of height axes
Push through or paddle off to begin vertical landing

Cons

Remembering to select height hold
More buttons required