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Created attachment 18002 [details]
Example project that reproduces the bug.
Sometimes, when you add or remove items from a ListView, the Header element is given an incorrect size and disappears. I'm not exactly sure what sequence of events causes it, but I have attached a project that reproduces based on the UI in my project.
The project represents a page that is performing some long operation that will produce a set of items for the ListView. In preparation, the Header element of the ListView is set up. A 3-second Task inside the RouteDetailsView constructor simulates the completion of that long-running task and population of items. When the items are populated, the header disappears.
I've done some debugging via Immediate/Watch and discovered the Label in the header is being given a Height of -1. I've confirmed this with Label-derived classes that override OnSizeAllocated(). My understanding is you should NEVER pass a -1 for any size parameter in XF layout, so something's probably going haywire in the renderer for the ListView.
This is an iOS-specific bug I have witnessed on 9.3.4 and iOS 10 devices and simulators. It does not reproduce in any Android cases I tried. I'm using Xamarin Studio 6.1.1 with Xamarin.iOS 10.0.1.10. I first experienced the bug in Xamairn.Forms 184.108.40.20624, but I have also reproduced it in 220.127.116.11.
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