What i think is happening is that, since you want to update the images all the time, you're basically resetting the "viewport" even if your slider has a viewport of the background image which is 50%, so the glitches you see should be happening around each time you change numbers on the knob. I've verified this in your project.
I guess it's not designed to be used that way. You could create a specialized class that can change only the knob image for the slider and does nothing else. Currently you can only set all the images at once, because you're not expected to change them after init.
What i think is happening is that, since you want to update the images all the time, you're basically resetting the "viewport" even if your slider has a viewport of the background image which is 50%, so the glitches you see should be happening around each time you change numbers on the knob. I've verified this in your project.
I guess it's not designed to be used that way. You could create a specialized class that can change only the knob image for the slider and does nothing else. Currently you can only set all the images at once, because you're not expected to change them after init.
