Inter Item Gap Indicator Not Visible on Horizontal Scrolling NSCollectionView layouts

Originator:markbridges9
Number:rdar://33554885 Date Originated:27/07/17
Status:Open Resolved:
Product:AppKit Product Version:macOS 10.12.6 (16G29)
Classification:UI Reproducible:Always
 
AppKit

Summary:
When I set the scroll direction on an NSCollectionViewFlowLayout to NSCollectionViewScrollDirectionHorizontal the inter item gap indicator no longer appears during dragging operations.

Steps to Reproduce:
Download the Apple provided sample code for the new iOS 10.11 NSCollectionView API.
https://developer.apple.com/library/content/samplecode/CocoaSlideCollection/Introduction/Intro.html

In AAPLWrappedLayout.m add the following at line 22.
[self setScrollDirection:NSCollectionViewScrollDirectionHorizontal];

Run the app and select the wrapped layout.
Drag a cell to reorder.

Expected Results:
A vertical blue line should appear to indicate the drop location, as per it does without the aforementioned​ line added

Observed Results:
No line appears

Version:
macOS 10.12.6 (16G29)

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