This test copies and pastes content inside pre that is an editing host. WebKit should not clone pre.
To manually test, cut and paste "hello\nworld" WebKit should not nest pre (no red borders).

Before cut paste:
| <pre>
|   "<#selection-anchor>hello"
|   <br>
|   "world<#selection-focus>"

After cut paste:
| <pre>
|   "hello"
|   <br>
|   "world<#selection-caret>"