I noticed an issue where all of my documents disappeared even though I cancelled the close (CanCloseAsync returns false). When a document closes, it prompts the user to save (Yes, No, or Cancel) where cancel stops the document from closing. I had upgraded several components including AvalonDock, Gemini, and Caliburn.Micro, so I wasn't sure where the error originated. I'm fairly sure the issue is in AvalonDock.

I created a test repo to demonstrate the issue: https://github.com/ryanvs/AvalonDockIssue - I also compared AvalonDock to a standard TabControl.

I haven't figured out exactly what is happening, but after the close is stopped, the PART_SelectedContentHost is empty and the ActualWidth is 0.