Currently, if any Data-driven Collection Elements are using the 'Selected Index' Property, adding a new Data Property to a Structure used by the Project, will force a reset of the 'Selected Index' Property value until the Page is closed and re-opened.

See attached video.

Repro Steps:

1) Download, unzip, and open the attached Project, and open MainPage.xaml contained within the Project.

2) Select the Character Grid ListBox on Stage, and expand its Properties to reveal its Selected Index Property.
Observe how the Character Grid (set to Index '0'), and the Stats Panel to the right of it display a selected Character from the grid.

3) Navigate to the Data Panel, select the 'CharacterData' Data Structure, and in its Properties, add a new Data Property.

Observe how upon creation of the new Data Property, entire pieces of the interface display from the Stage.

5) Select the Character Grid ListBox on Stage once again, and expand its Properties to reveal its Selected Index Property.
Observe how its SelectedIndex has been changed to -1 with no user action inputting this new Property Value.

6) Close and re-open MainPage.xaml

Observe how the Selected Index of the ListBox, has returned to the expected value of '0', and the interface once again displays as expected on the Page.

Expected Results: Adding new Data Properties does not affect the Properties or presentation of the UI on the Page.