Open Radar

 
Summary:
When animating the setting of the NSPageController selectedIndex property, if you are using the NSPageControllerTransitionStyleHorizontalStrip transition style, the animation will sometimes look like a stack instead of a horizontal strip.

Steps to Reproduce:
1. Download the FileCards sample code:
https://developer.apple.com/library/prerelease/content/samplecode/FileCards/Introduction/Intro.html
2. In the project, open MainMenu.xib.
3. Click on "PageController" in the Objects list.
4. In the Attributes inspector, set the page controller's transition style to Horizontal Strip.
5. Build and run. (Note: You will need at least two files in your ~/Documents folder to proceed.)
6. In the Files list in the app, click on any item that is not currently selected. Click on the Back or Forward toolbar button to change pages again. You may need to repeat this step 2-3 times.

Expected Results:
Because the transition style is Horizontal Strip, every page change should consistently animate with a horizontal slide. No page should ever appear under another page, because the transition style is not a stack style.

Actual Results:
Eventually, a page change will animate like a card stack, with one page sliding out from underneath the other. Sometimes the animation will be a horizontal slide as expected, but other times not. It's inconsistent.

Version:
Xcode 7.3.1 / macOS 10.11.6

Notes:
The only workaround is to not animate at all, which is not ideal.