Performance Improvement Request: horizontal items

Nobody has claimed this yet.

### Description

Whenever you use a horizontal item in the view, `Jasonette[10916:7536476] NSScanner: nil string argument` appears in the log.

I found that this was related to my observation that having a horizontal item within the view causes some jitter when scrolling _vertically_ in the view, which isn’t so noticeable on a mac but it is on a phone. Whether the horizontal item scrolls or just stretches to fill-up the view horizontally is immaterial.

### Example Code

Control (no horizontal items): https://jasonbase.com/things/rykb.json
Example (horizontal items, jittery when scrolling down): https://jasonbase.com/things/ePj7.json

Interestingly horizontal components aren’t affected; only appears to affect items.

Start by reproducing the issue with the provided control and horizontal-item examples at jasonbase.com, comparing vertical scrolling and the NSScanner log output. Trace the horizontal item path and compare it with horizontal components. Done means horizontal items no longer produce the warning or noticeable vertical-scroll jitter on a phone.

Written by the indexing model from the issue text.