STR:

(Optional) 1. Watch user attached video where problem is explained in detail https://youtu.be/gFRwVKEIci4
2. Open user attached project
3. Build & deploy to iOS device
4. Drag around grey area of the screen to see current touch position change rate. It should be around (30Hz) but this is not a fact;
5. Click on input area on top left corner of screen.
6. Dismiss keyboard
7. Drag around grey area of screen

Expected result: dragging ScrollBar is always smooth, touch position change rate is always at 60Hz
Actual result: Smoothness of scrollbar dragging is affected by opened dialogs and is varying between 30Hz and 60Hz.

Note: It seems that these steps are not 100% accurate and might differentiate by device. Point is that opening and dismissing different dialog boxes from top of the screen affects touch position change rate.

Reproduced with: 5.5.0a6, 5.5.0b9;

DUT:
Reproduced with:
iPhone 6+(10.0);
Did not reproduce with:
iPhone 6(9.0.2);