This problem does not occur when running PIE with New Editor Window mode.
1. create a project
2. create a widget blueprint based on the user widget in the content browser
3. open the WidgetBlueprint
4. change to graph view
5. create any node (e.g. Print string) by dragging it from the Event Tick node
6. select the newly created node and press F9 to place a breakpoint
7. open the level blueprint
8. instantiate the widget by placing Create Widget and Add to Viewport
9. start PIE in Selected viewport mode
[Link Removed] is the repro project
The whole editor rendering is blocked. It seems to freeze but editor is still running(tick is spinning) and you can press the Esc key or the Stop Debug button.
There's no existing public thread on this issue, so head over to Questions & Answers just mention UE-197297 in the post.