There are two issues here, sort of packed into one. First: logic around _.slideCount % _.options.slidesToScroll !== 0 makes the slider jump randomly in some cases after animation. Second: the logic for cases around _.slideCount % _.options.slidesToScroll !== 0 seem to cause behavior that was decided arbitrarily. For instance,

if (targetSlide >= _.slideCount) {
            if (_.slideCount % _.options.slidesToScroll !== 0) {
                animSlide = 0;
            } else {
                animSlide = targetSlide - _.slideCount;
            }

why would we always want to start back over at the first slide when we cycle through pressing Next?

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https://codepen.io/samjacobs/pen/RwrRdxR

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Steps to reproduce the problem

a)
When using Infinite mode where _.slideCount % _.options.slidesToScroll !== 0, say 10 slides, slidesToScroll === 3,
Given the user presses previous once,
Then the math for the animation in _.getLeft does not match the math at the end of the animation from _.setPositioon, so the slider jumps to a different spot.

b)
When using Infinite mode where _.slideCount % _.options.slidesToScroll !== 0, say 10 slides, slidesToScroll === 3,
Given the user presses next to go from slide 7 to slide 10,
Then the slider animates to slide 5.

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What is the expected behaviour?

a) Pressing previous from the first slide, with slidesToScroll being 3 and 10 slides, , I would expect to jump to slide # 8.

b) I would the slides to animate to slide 10, and then pressing again to animate to slide 3.

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What is observed behaviour?

a) The slides animate to the correct slide, then jump to slide 5.
b) The slide animates to slide 5.

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More Details

• I discovered this using the latest version of react-slick.
• Can be reproduces using 3.5.1 jquery and 1.9 slick.js in codepen