What is Jelly Scrolling you ask? It's an effect caused by the inversion of the display in certain phones - right now the OnePlus 5 and the Moto Force Z2, that makes the elements in the page become larger and smaller while scrolling (like a jelly). It is now always noticeable, but it's most prominent with images, not so much with text. Some users might never see it. It depends of your perception. Also check out my review of the OnePlus 5 and why I will be returning it.

How has it come to this? In order to save interior space and make the phone thinner, OnePlus have mounted the display inverted, so that the connector pins are at the bottom and not the top how they are designed to be. The top of the OnePlus 5 is pretty crowded with the addition of the dual camera and probably the engineers tough it would be a good idea to mount the display up-side down and change it's settings from the software to display correctly - 180 degrees switch. Unfortunately, because of V-SYNC issue and the fact that now the display is refreshed from the bottom to the top as opposed to its designed way from top to bottom, jelly scrolling is present.

 
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The issue was also confirmed on the famous XDA Developers website, they have a whole article about this.

One Plus have also responded to this issue by issuing a controversial statement: 

"The OnePlus 5 uses the same level of high-quality components as all OnePlus devices, including the AMOLED display. We’ve received feedback from a small number of users saying that at times they notice a subtle visual effect when scrolling. This is natural and there’s no variance in screens between devices."

This is basically not acknowledging fault for the Jelly Scrolling on the OnePlus 5 and since this is a hardware issue, no updates will be able to solve it. Users can return the phone after 15 days, as per the company policy tough. This is what I will be doing of course and I suggest you do it too, just to be safe, you can never know how an issue like this can affect your vision long term.

Proof that the display is mounted upside down:

OnePlus 5 display mounted upside down

Source: https://www.reddit.com/user/Tasssadar

And the subsequent software change to make the display right again, found in the kernel:

Source: XDA

You can try this effect on any phone, just hold it upside down and scroll. i can confirm it is visible on my Samsung Galaxy S7 Edge when held upside down.