1. Go to MLibTest project.
2. Comment in MainWindow.xaml.cs "stringLayoutSerializer.LayoutSerializationCallback += (s, e) =>..." and the following lines with event callback (lines 80-114). (This callback may be absent in real application and will not be called by LayoutSerializer in FixupLayout.)
3. Compile the project.
4. Go to compiled application folder and create AvalonDock.Layout.config file with the content of the attached file. Please note that it contains LayoutDocument element with ContentId applied.
5. Run MLibTest.exe and close it without performing any actions.
6. AvalonDock.Layout.config was updated and does not contain LayoutDocument.

Desired result: LayoutDocument element with ContentId sholud be kept between sessions.

AvalonDock.Layout.txt