I've looked for this in Preferences but perhaps missed it. Is it possible to turn self-test off?
The problem I've encountered is possibly rare. In a notebook I have a cell containing formatted text. The style for the cell is Subsubsection but it also includes widgets from Palettes ⤑ Basic Math Assistant ⤑ Typesetting. The primary one is an arrow with before, after, and above blocks. Within the before and after blocks I have placed columnar blocks, and within those Text. The problem only occurs when I select some or all of the text in any of the left-hand blocks. Editing (insert, delete single characters) works fine. Upon selection though, the Messages dialog pops-up and the entire session freezes for hours with no recovery. My work-around is to kill the process and restart Mathematica.
Here's an example notebook:
