I did try the GTK2 Appimage. Just the same behavior. The link you referenced above for a commit seems like describing a similar problem.
All sliders in my other applications run normally like those displayed by “gtk3-widget-factory”.
ADD: I have found the “Natural scrolling” adjustment. If I turn it ON, PhotoFlow sliders behave NORMALLY! But, darktable and GIMP start MISBEHAVING.