Currently, if you create a Run within a TextBlock, it locks out editing of both the overall TextBlock, as well as editing of the Runs created.
As a consequence of this, further Runs are also not able to be created from the original TextBlock.

See attached video.

Repro Steps:

1) Create a Project with a MainMenu.xaml Page
2) Introduce a TextBlock to the Page's root Grid, and write a sentence in the 'Text' Property field.
3) On the Stage, double-click on the TextBlock that now contains the text written in Step 2). This will display the TextBlock's adorner.
4) Using click-and-drag, highlight a single word within the sentence, then press the (+) icon in the adorner to create a Run from this TextBlock.
5) Attempt to use click-and-drag of the mouse to select another word within the sentence to convert into a Run.

Observe how further text cannot be selected, and therefore further Runs can't be created.
Observe how at the Stage-level, none of the text can be edited further.

Expected Results: The TextBlock is able to be continuously edited, and further runs are able to be created beyond the first Run created (as perfomed in Step 3) and Step 4))