UIPageViewController may crash when scrolling between zoomed pages

Originator:stevjerbs
Number:rdar://19269483 Date Originated:16-Dec-2014
Status:Open Resolved:No
Product:iOS SDK Product Version:iOS 8.1
Classification:Crash Reproducible:Rarely
 
Summary:
UIPageViewController can crash with "NSInternalInconsistencyException: Failed to determine navigation direction for scroll" when scrolling back and forth between zoomed-in UIWebViews (or possibly scroll views) as pages.

Steps to Reproduce:
1. Create a UIPageViewController with UIWebViews that can be zoomed in.
2. Pinch zoom in on the current page such that a horizontal scroll bar appears.
3. Go to an adjacent pages and pinch zoom on that one as well.
4. Pan edge-to-edge between this page and the other zoomed page (without lifting the finger).

Expected Results:
UIPageViewController shouldn't crash

Actual Results:
UIPageViewController internally crashes

Version:
iOS 8.1

Notes:
Attached is a basic sample project, the stack trace, and a small video showing the workflow in an iOS 8 simulator.

Configuration:
The New iPad with iOS 8.1.2, iOS 8 simulator

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